When you rewrite fractions with a common denominator, you must create equivalent fractions. Do not change only the denominator.

$\displaystyle\frac{{7}}{{9}}-\frac{{1}}{{4}}=\frac{{28}}{{36}}-\frac{{9}}{{36}}=\frac{{19}}{{36}}\usepackage{cmbright}\Huge\$

The correct answer is $\displaystyle\frac{{19}}{{36}}\usepackage{cmbright}\Huge\$ .