one way to solve it is writing two equations based on the info so
y = 5x + 10 for members
y = 6x for nonmembers
(the + 10 isn't included in the second equation because nonmembers won't pay a membership fee)
and to find their intersection point (where the values will be the same), you just set them equal to each other:
5x + 10 = 6x
subtract 5x
x = 10 classes