Setting up myscripts.js files

Hello. I would like to know how to call a Java script functions from a 'myscripst.js' file and use it in the mortgage_form.htm page. The pages are supplied below. I have read that this is not difficult but I can no seem to get it to work.
Thank you in advance
Clive Foyster


/\/\/\/\/\/\/\/\/\/\/\/\ = mortgage_form.htm
<table>
  <tr>
    <td><form method="POST">
      <b><p>Mortgage Calculator</b></p>
    </form>
    </td>
  </tr>
  <tr>
    <td>Principal: </td>
    <td>$ </td>
    <td><input TYPE="TEXT" NAME="principal" SIZE="9" onChange="computeField(this, false)"></td>
  </tr>
  <tr>
    <td>Interest: </td>
    <td></td>
    <td><input TYPE="TEXT" NAME="interest" SIZE="9" onChange="computeField(this, false)">%</td>
  </tr>
  <tr>
    <td>Duration:</td>
    <td></td>
    <td><input TYPE="TEXT" NAME="years" SIZE="9" onChange="computeField(this, false)">yrs</td>
  </tr>
  <tr>
    <td><input TYPE="reset" VALUE="Reset" onClick="clearForm(this.form)"></td>
    <td></td>
    <td><input TYPE="button" VALUE="Calculate" onClick="computeForm(this.form, true)"></td>
  </tr>
  <tr>
    <td>Payment:</td>
    <td>$</td>
    <td><input TYPE="TEXT" NAME="payment" SIZE="9" onChange="computeField(this, false)">/mth</td>
  </tr>
</table>


\/\/\/\/\/\/\/\/\/\= myscripts.js
script for mortgage calculation
<script LANGUAGE="JavaScript" name="mortage_calc">
<!--

function checkNumber(input, min, max, msg)
{
    msg = msg + " field has invalid data: " + input.value;

    var str = input.value;
    for (var i = 0; i < str.length; i++)
      {
        var ch = str.substring(i, i + 1)
        if ((ch < "0" || "9" < ch) && ch != '.')
            {
            alert(msg);
            return false;
        }
    }
    var num = parseFloat(str)

    if (num < min || max < num)
      {
        alert(msg + "\n must be between " + min + " and " + max);
        return false;
    }
    input.value = str;

    return true;
}


function computeField(input, buttonFlag)
{
    if (input.value != null && input.value.length != 0)
        input.value = "" + eval(input.value);
    computeForm(input.form, buttonFlag);
}


function computeForm(form, buttonFlag)
{
    if((form.years.value == null || form.years.value.length == 0) ||
         (form.interest.value == null || form.interest.value.length == 0) ||
         ((form.principal.value == null || form.principal.value.length == 0) &&
         (form.payment.value == null || form.payment.value.length == 0)))
      {
          if(buttonFlag == true)
          alert("Please supply enough information to calculate.")
       
            return;
    }

    if(form.years.value != null && form.years.value.length != 0 &&
         form.interest.value != null && form.interest.value.length != 0 &&
         form.principal.value != null && form.principal.value.length != 0 &&
         form.payment.value != null && form.payment.value.length != 0)
            form.payment.value = "";      

      var i = form.interest.value / 1200.0;
      
      var pow = 1;
      for (var j = 0; j < form.years.value*12; j++)
            pow = pow * (1 + i);

      if(form.principal.value == null || form.principal.value.length == 0)
      {
            if (!checkNumber(form.years, 1, 30, "# of years") ||
                  !checkNumber(form.interest, .001, 99, "Interest") ||
                  !checkNumber(form.payment, 1, 100000, "Payment"))
            {
                  form.principal.value = "Invalid";
                  return;
            }
         
          form.principal.value = Math.round((form.payment.value * (pow-1) ) / (pow * i))
      }
      else if(form.payment.value == null || form.payment.value.length == 0)
      {
            if (!checkNumber(form.years, 1, 30, "# of years") ||
                  !checkNumber(form.interest, .001, 99, "Interest") ||
                  !checkNumber(form.principal, 100, 10000000, "Principal"))
            {
                  form.payment.value = "Invalid";
                  return;
            }

            form.payment.value = Math.round(100.0*(form.principal.value * pow * i) / (pow - 1)) / 100.0
      }
}


function clearForm(form)
{
    form.years.value = "";
    form.interest.value = "";
    form.principal.value = "";
      form.payment.value = "";
}

//-->
</script>