What rent should be charged to obtain a maximum profit? Further information below!

Question

A real estate office handles a 80-unit apartment complex. When rent is $550 per month, all units are occupied. For each $40 increase in rent, however, an average of one unit becomes vacant. Each occupied unit requires an average of $60 per month for service and repairs. What rent should be charged to obtain a maximum profit?

Just a quick note: I got $1905 but my answer was wrong.

Answer 1

i represents the number of rent increases

R(i) = (550 + 40i)(80 - i)

R(i) = 44000 +2650i - 40i²

RIi) = - 40i² + 2560i + 44000

Maximum occurs at x = -b / 2a