81x-29y because you do 65x plus 16x because they both have the variable x at the end. now do 50y plus -21y and since it is a negetive (and it is a negetive it is not just a subtraction sign trust me i have done this many times before and been correct) you get 29 y because they both have the variable y in it. and you can't do anything with a two different variables.
