Alan,
This is like adding apples and oranges  we can put apples together, and we can put oranges together, but we can't put apples and oranges together because they are not the same. For example, if we have 7 apples and 8 oranges, and then we gain 2 apples and lose 6 oranges, we will end up with 9 apples and 2 oranges. That is,
7 apples + 8 oranges + 2 apples  6 oranges = 9 apples + 2 oranges.
In the question below, the x and y are unknown variables, so we can't put together the x's and y's, just like we can't put together apples and oranges, because we don't know if the x and y are the same (we must assume they are different). Thus,
7x + 4y + 2x  6y
= 7x + 2x + 4y  6y
= 9x  2y
Paul
