I am new to this. I am trying to teach myself algebra and physics. I am trying to understand the last part of this under

*relating the objects*. All circled variables are unknowns. More specifically part 3 and 4 especially 4, using a quadratic formula to figure out the unknown for t (

*time*).

1. Can you provide some clarity how the author found the unknowns when there are multiple unknowns in each equation? I have read there are ways to solve for two unknowns, but in this case it's a little foggy to me.

2. When do I know a situation calls for the use of the quadratic equation? I had no idea to use that here.

t^2-18t-20=0

I would not have known what numbers and variables especially

*t^2* to insert into quadratic equation.

Thanks.

plong.jpg
2. http://www.purplemath.com/modules/quadform.htm