The surface area of a cube is just the area of all six sides of the cube. Therefore, if we let s = the cube's side,
The surface area would be 
6
⋅
s
2
 because the area of one face is 
s
2
 and there are six faces
Given this, we know from the problem that 
s
=
5
We can just plug in for s.
SA = 
6
⋅
5
2
SA = 
6
⋅
25
SA = 150