We first factor the numerator (top) then cancel.

`{:((x^2-y^2)/(y-x),=((x+y)(x-y))/(-(x-y))),(,=(x+y)/-1),(,=-x-y):}`

On the first line of the solution, I am changing the order of the (y − x) into (x − y) by placing a minus in front.

This is because

−(x − y)

= −x − (-y)

= −x + y

= y − x