5x + 5= 15x - 12 - 10x
5x+ 5= 5x - 12
You add the 15x and -10x which will give you a 5x then you minus the 5x from the left or right to the other 5x, but it doesn't really matter which 5x you minus (or use) then you will be left with 5 = -12 which gives you a no solution as your answer.