X * (1 + P)^N = Y

If you divide both sides by X, take the natural logarithms, and use the logarthm property that

ln(a^b) = b * log(a)

you have:

N * ln(1 + P) = ln(Y/X)

Now you need to move N to the right and take the exponential of both sides, because it is the inverse function of the natural logarithm:

exp(ln(1 +P)) = exp(ln(Y/X)/N)

which finally gives you:

P = exp(ln(Y/X)/N) - 1

In Excel this formula is written as follows:

=EXP(LN(Y/X)/N)-1

where Y and X are the cells where your goal and the March revenue are stored. Obviously, you can also store your goal directly in the formula:

=EXP(LN(1000000/B15)/N) - 1

If you want to have a parametric N, you can use the function COUNTA(cell-range) which counts the non-empty cells in a range. Therefore, COUNTA(A:A) would count the months. Another useful function in this context might be ROW(reference) which gives you the row of a cell. Therefore:

COUNT(A:A) - ROW(B15)

would calculate for you the number of months left (72 - 15)

There are perhaps financial functions which would be useful, but I am familiar with the Math functions...