ok, let's simplify the left side first by adding the x's together
We get:
12x+5=12x+y
So if we subtract 12x from each side, it cancels it out on both sides, leaving us with 5=y or y=5
If we plug that back into the original we simplified of 12x+5=12x+y, we would have: 12x+5=12x+5, which is true. :D