Take (x+y) as a whole, such as z, then the equation becomes z times (Z+ 1)=20.

Take it inside. This is an unary quadratic equation. Just solve the quadratic equation of one variable.

Multiply by the cross.

(Z-4)(Z+5)=0 so Z= 4 or -5.

That is, x+y = 4 or -5.

-5 does not meet the meaning of the question (omitted)

It should be like this. Ha ha laugh