$$\frac{x}{21y-y^{2}}+\frac{y}{xy-x^{2}}$$
$\frac{yx^{2}+21y^{2}-y^{3}-x^{3}}{xy\left(21-y\right)\left(y-x\right)}$
$\frac{x^{4}-2xy^{3}+42xy^{2}+y^{4}-21y^{3}-2yx^{3}+\left(xy\right)^{2}}{y\left(21-y\right)\left(x\left(x-y\right)\right)^{2}}$
