8 + x - 8 = 13 + 2x - 8
The 8 cancels on the left
x - 2x = 13 + 2x - 8 - 2x
The 2x cancels on the right
x - 2x = 13 - 8
Do the calculation to find
- x = 5
- x . (-1) = 5 . (-1)
Times by -1 to cancel the negative sign because you arises to find x and not -x
You finally get x = -5