X^2 - 5x - 24 is the answer
1). Make your equation equal to 0
2). Then plug in your numbers to the quadratic equation
Sometimes it will, and sometimes it won't.
I reason as follows:
(21 + x) + (30 + 2x) = 51 + 3x has an 'x' term
(42 + x) + (30 - x) = 72 has no 'x' term.
This is simple division:

So, plug it into your calculator and you get:

Your answer's that
it takes 4.5 hrs for the surveillance run.