First you would combine like terms on each side of the equation It would look like this... 4X+8= 4X-13
Then you would add 13 to both sides, the 13 on the right side of the equation would cancel out and the 8 would become 21. It would look like this... 4X+21=4x.
Now you would subtract 4X from both sides. The 4's Would cancel out which would leave you with 21=X
