Answer
$\frac{13n}{24}$
Work Step by Step
By inspection, we see that the LCM of the denominators is $72$. Therefore, we create a common denominator using this LCM and obtain:
$\frac{7n}{8}-\frac{3n}{9}$
=$\frac{9(7n)-8(3n)}{72}$
=$\frac{63n-24n}{72}$
=$\frac{39n}{72}$
=$\frac{13n}{24}$
