I know that the formula for finding the surface area of a rectangular prism is 2ab + 2bc + 2ac, but how do I "fill in the variables", with knowing the correct numbers to solve the problem?

You're right but just to simplify your life, make the formula this ...

A=2 (wl+hl+hw)

w=width

h=height

l=length

Easy, just fill in the "variables."

Example,

l=2

w=2

h=2

Plug it in ...

2 (2*2+2*2+2*2)

2 (4+4+4)

2 (12)

24

Thus, your surface area example ...

Easy.