Given, x4+x2y2+y4=21 and x2+y2+xy=3 We can write as x4+y4+2x2y2−x2y2=21 ⇒(x2+y2)2−x2y2=21 . . . (i) and x2+y2+xy=3 ⇒x2+y2=3−xy Now, Eq. (ii) put in Eq. (i), ⇒(3−xy)2−x2y2=21 ⇒9+x2y2−6xy−x2y2=21 ⇒9−21=6xy ⇒−12=6xy ⇒xy=−2 ∴(−xy)=−(−2)=2