To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x^{3}$ and $y^{3}$ is $x^{3}y^{3}$. Multiply $\frac{1}{x^{3}}$ times $\frac{y^{3}}{y^{3}}$. Multiply $\frac{1}{y^{3}}$ times $\frac{x^{3}}{x^{3}}$.
