simplify the question with the restriction

Question

x^2+9y^2/x-3y + 6xy/3y-x

Answer 1

Assuming the usual carelessness with parentheses, we have
(x^2+9y^2)/(x-3y) + 6xy/(3y-x)
= (x^2-7xy+9y^2)/(x-3y)
= (x-3y)^2/(x-3y)
= x-3y
as long as x-3y ≠ 0