Question

$sqrt{7} x(sqrt{7}xy-x)-(x-y)(sqrt{28}x+y)-(sqrt7xy)^2$

Answer 1

$...=7x^2y- sqrt{7} x^2-2sqrt{7}x^2+2sqrt{7}xy-xy+y^2-7x^2y^2=\=-7x^2y^2+7x^2y-3sqrt{7}x^2+xy(-1+2sqrt{7})+y^2$