$x^2+y\sqrt{xy}=336 \quad ...(1)$
$y^2+x\sqrt{xy}=112 \quad ...(2)$
Dividing (1) by $\sqrt{x}$,
$x\sqrt{x}+y\sqrt{y}=\frac{336}{\sqrt{x}}$
Dividing (2) by $\sqrt{y}$,
$y\sqrt{y}+x\sqrt{x}=\frac{112}{\sqrt{y}}$
Since LHS are equal,
$\frac{336}{\sqrt{x}}=\frac{112}{\sqrt{y}}$
$\frac{\sqrt{x}}{\sqrt{y}}=3$
$\sqrt{x}=3\sqrt{y}$
$\therefore x=9y$
Substituting in (2),
$y^2+9y\sqrt{9y^2}=112$
$y^2+27y^2=112$
$28y^2=112$
$y^2=4$
$\therefore y=2$
$x=9(2)=18$
$\therefore \boxed{x+y=20}$