a club has 300 members who pay 400 a month, the club wants decrease by 1 dollar, what price should be charged for maximum revenue

Question

they want to decrease one dollar in the monthly membership so 2 more people will join because thats what research showed. what price should the club charge to maximise the value 

Answer 1

r represents revenue

d represents each decrease

r = (300 + 2d)(400 - d)

 

Can you convert to vertex form and answer?