x=2y and x^2+2x-y-y^2=36

x=2y (1)

x^2+2x-y-y^2=36 (2)

Replace x in (2) by 2y

4y^2 +4y -y - y^2 = 36

3y^2 + 3y = 36

y^2 + y = 12

y^2 + y - 12 = 0

(y + 4)(y - 3) = 0

Hence (i) y = -4, and using (1) x = 2*(-4) or x = -8

and (ii) y = 3, and using (1) x = 2*3 or x = 6