~~(a - b)~~(a + b) = b~~(a - b)~~

However, this results in dividing by , because

EDIT:

Your mistake comes in step 5, when you divided by (x-y) on both sides. Since x = y, x-y = 0, so you are dividing by zero, which is undefined (illegal!). Source: http://mathforum.org/library/drmath/view/55792.html