.
(a) The profit is the revenue minus the cost
P(x) = (75x - 0.5x^2) - (35x + 300) = -0.5x^2 + 40x - 300.
(b) For it, solve the quadratic equation
P(x) = 50, which is -0.5x^2 + 40x - 300 = 50.
-----------
When a person gets such a problem, it is assumed that he knows how to solve quadratic equations.
If, in opposite, you don't know how to solve them, consult these lessons
- Introduction into Quadratic Equations
- PROOF of quadratic formula by completing the square
in this site.