First we have to find the equation in which there are maximum numbers of passengers in the bus during certain numbers of stop.

P' = -2(2)(s-2)
= -4s + 16

Maximum number of passenger happens when P' = 0

0 = -4s + 16
s = 4

So maximum passenger is reached during the 4th stop.

The question's asking for two stops after the stop where it has its greatest number of passengers, i.e. during the 6th stop. So plug-in the number:

P = -2(6-2)^2 + 32
= -2(4) + 32
= 24

I hope that helps.