To simplify expressions, you have to be able to combine like terms (eg. 3x and 2x to get 5x). Since there are none in this expression, and you can't combine y and x, this expression can't be simplified.
*If it helps, think of x and y as different types of candy... ;3
For example, you can't combine 12 Skittles (x) and 4 Reese's (y), because they aren't the same. So, you'd write that you have 12 Skittles and 4 Reese's.
