Answer :

AL2006
Fact #1:
A cube has six faces.

Fact #2:
Every one of them has the same area.

Fact #3:
The surface area of each face is (the length of the cube's side)²

Now that I look at this one more carefully, the only thing you need to know
to solve it is Fact #3.  (But you can keep the others and save them for later.)

The surface area of each face is (the length of the cube's side)²

64 is the (length of the cube's side)²

Can you find a number that gives you 64 when you multiply it by itself ?
That's the length of the cube's side.

Hint: If you don't get ' 8 ', work on it a little more.