To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $9x^{2}$ and $25y^{2}$ is $225x^{2}y^{2}$. Multiply $\frac{121}{9x^{2}}$ times $\frac{25y^{2}}{25y^{2}}$. Multiply $\frac{4}{25y^{2}}$ times $\frac{9x^{2}}{9x^{2}}$.
Since $\frac{121\times 25y^{2}}{225x^{2}y^{2}}$ and $\frac{4\times 9x^{2}}{225x^{2}y^{2}}$ have the same denominator, subtract them by subtracting their numerators.
