If $x^4+x^2 y^2+y^4=21$ and $x^2+x y+y^2=3$, then what is the value of $(-x y)$ ?

-1 2 1 -2

2

x4 + x2y2 + y4 = (x2 – xy + y2) (x2 + xy + y2) x4 + x2y2 + y4 = 21 and x2 + xy + y2 = 3      --- (1) = 21 = (x2 - xy + y2) × 3 = (x2 - xy + y2) = 7      --- (2) From equation (1) from equation (2) 2xy = -4 xy = -2 -xy = 2