x+y is NOT the same thing as xy, x+y doesn’t mean we simply get to put the variables together to get xy since they are not like terms. xy is x multiplied by y. An example of combining like terms would be something like this, y+y which equals 2y. Or we could make it a little more difficult and have, 2x+y+x+2y, rewrite it like this => 2x+x+2y+y we get 3x+3y. Hopefully this little explanation helps a bit. If you have any questions comment them under this answer.