Ta có:
$\left\{\begin{array}{l}x^3-y^3=28\\x^2+3y^2=x-9y\end{array}\right.$
$\Leftrightarrow \left\{\begin{array}{l}x^3-y^3=28\\x^3-y^3-3(x^2+3y^2)=28-3(x-9y)\end{array}\right.$
$\Leftrightarrow \left\{\begin{array}{l}x^3-y^3=28\\(x-1)^3=(y+3)^3\end{array}\right.$
$\Leftrightarrow \left\{\begin{array}{l}x=y+4\\(y+4)^3-y^3=28\end{array}\right.$
$\Leftrightarrow \left\{\begin{array}{l}x=y+4\\y^2+4y-3=0\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}\left\{\begin{array}{l}x=3\\y=-1\end{array}\right.\\\left\{\begin{array}{l}x=1\\y=-3\end{array}\right.\end{array}\right.$