Answer :
The addition of polynomials follow commutative, where as the subtraction doesnt follow commutative.
3x + 5x = 5x + 3x = 8x
5x - 3x = 2x but 3x - 5x = -2x
3x + 5x = 5x + 3x = 8x
5x - 3x = 2x but 3x - 5x = -2x