Recall your d = rt, distance = rate * time.
notice, the distance are the same, say it was "d" miles.
and if car A is travelling at a speed of say "r" mph, then B is going at "r+15" mph.

how far is B going? well, r + 15.
Two solutions:
1. x =(100-√2160)/2=50-6√ 15 = 26.762
2. x =(100+√2160)/2=50+6√ 15 = 73.238
Here is an example on how you should do your multiplication. The website should give you your answer to any multiplication.
Hope this helps love!! <3