What is the 3rd perfect number and what are its divisors?

The graph below shows the amount of money left in the school’s desk fund, f, after d desks have been purchased. for each new desk that is purchased, by how much does the amount of money left in the school’s desk fund decrease?

Based on what i know about mean, this doesn't add up.

The parabola x= √y-9 opens: a.)up b.)down c.)right d.)left