Let P = price and C = customers.
The relationship between P and C is linear as follow:
slope = (36 -35)/(12-12.5)= -2 = (36-C)/(12-P)
Multiply and collect terms: C = 60 - 2P
Revenue = P*C = 60P - 2P2
Revenue is maximized when the derivative is 0 i.e. when 2P = 30 and P = 15 and C = 30
Revenue will be 450 and customers lost will be 6