Hey dumbblonde.

Basically it means that you make two constant, and change the other while plotting a few values of x.

So as an example, let a = 1 and c = 1. Then choose a value of b (say 1) and plot a few values of x to get the graph. Then pick a new b (say b = 2) and do the same thing.

Then just look at how the graphs change when you change the value of b.