Answer
$ \frac{189-9n}{7n^{3}}$
Work Step by Step
Given : $\frac{27}{n^{3}} - \frac{9}{7n^{2}}$
This becomes : $\frac{27\times 7}{n^{3} \times 7} - \frac{9 \times n}{7n^{2}\times n}$
(Finding the least common denominator)
$= \frac{189-9n}{7n^{3}}$
