There is an easier way to do the substitution, simply add the two equations. As to your mistake you used the first equation to solve for y in terms of x, so you have to plug in into the second equation instead of the first. Also you made an addition error because you should be getting 0=0 since you used the same equation you simplified to plug into. In l3 & l4 on the right, you are supposed to add +12 to both sides instead of subtracting, which will give u 5x-5x = 12-12 as answer. But like I said if you were to use your method, you better use the other equation to solve for the system of equation other than the one you simplified.