SAT Practice Test 3 - need help with a math question

Hello,

I am stumped on number 36 on the calculator OK part, and there is no explanation for the correct answer: 750.

In the xy-plane, if a point with coordinates (a, b) lies in the solution set of the system of inequalities below, what is the maximum possible value of b?

y    less than or equal to   -15x + 3000

y    less than or equal to     5x

Thanks!