You may have added the numerators and added the denominators. To add these fractions, rewrite each with the least common denominator, which is $\displaystyle{5}{x}\cdot{3}{y}={15}{x}{y}\usepackage{cmbright}\Huge\$ . Start with:

$\displaystyle\frac{{4}}{{{5}{x}}}+\frac{{7}}{{{3}{y}}}=\frac{{4}}{{{5}{x}}}\cdot\frac{{{3}{y}}}{{{3}{y}}}+\frac{{7}}{{{3}{y}}}\cdot\frac{{{5}{x}}}{{{5}{x}}}\usepackage{cmbright}\Huge\$

Simplify the numerators and add them.