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