First, you distribut the x on the right side.
So, it would be 8x-5=3x+21. You want to get rid of one x on either side.
Take away the smaller one so you don't end up with negatives (which is harder to solve) So, 5x-5=21.
Add 5 to both sides. 5x=26.
Divide by 5. You get x=5 1/5.