If you are solving for x, you could set it up like this:
x + 8 = 2x + 5
-5 - 5
x + 3 = 2x
-x -x
3 = x
So now that you have the value of x. You can plug it in. The first one is x + 8 or 3 + 8 since the value of x is 3. Then the next one is 2x + 5 or 2(3) + 5.