$x^3+y^3 +6xy=21$
$\Leftrightarrow (x+y)^3 -3xy(x+y)+6xy -21=0$
$\Leftrightarrow \bigg [ (x+y)^3 - 8 \bigg ] -3xy (x+y -2) -13=0$
$\Leftrightarrow (x+y-2) \bigg [ (x+y)^2 +2(x+y) + 4 \bigg ] -3xy (x+y -2) -13=0$
$\Leftrightarrow (x+y -2) \bigg [ (x+y)^2 +2(x+y) + 4 -3xy \bigg ]=13$
Xong. Tự làm nốt đi