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{{6}}{{{7}{x}}}+\frac{{3}}{{{2}{y}}}=\frac{{6}}{{{7}{x}}}\cdot\frac{{{2}{y}}}{{{2}{y}}}+\frac{{3}}{{{2}{y}}}\cdot\frac{{{7}{x}}}{{{7}{x}}}\usepackage{cmbright}\Huge\$

Multiply, and then add the numerators.