It appears that you attempted to rewrite the fractions with the same denominator, but multiplied the numerators by the wrong factors. Start with:

$\displaystyle\frac{{1}}{{{3}{x}}}+\frac{{5}}{{{7}{y}}}=\frac{{1}}{{{3}{x}}}\cdot\frac{{{7}{y}}}{{{7}{y}}}+\frac{{5}}{{{7}{y}}}\cdot\frac{{{3}{x}}}{{{3}{x}}}\usepackage{cmbright}\Huge\$

Simplify the numerators and add them.