Answer
$-\frac{1}{x^{2}}$
Work Step by Step
By inspection, we see that there is a common denominator of $4x^{2}$. Therefore, we obtain:
$\frac{9}{4x^{2}}-\frac{13}{4x^{2}}$
$=\frac{9-13}{4x^{2}}$
$=\frac{-4}{4x^{2}}$
$=\frac{-1}{x^{2}}$
$=-\frac{1}{x^{2}}$
