The original rent is $400 and "for each $10 per month increase, there will be two vacancies with no possibility of filling them". If x is the monthly rent, then x- 400 is the amount of increase above $400 and (x- 400)/10 is the number of "$10 per month" increases. Since the rental of two apartments is lost for each such increase there will be 2(x-400)/10= (x- 400)/5 fewer apartments rented. That is, with rent at x, n(x)= 100- (x- 400)/5= 500/5- (x- 400)/5= (100- x)/5 apartments will be rented, each for x dollars per month. Now can you find R(x)? That will be a quadratic so you can find its vertex, answering (c), by completing the square.