Answer
$\frac{-17}{24n}$.
Work Step by Step
In order to add or subtract fractions, we start by making a common denominator (bottom of the fraction). Thus, we multiply 1/6n by 4/4 and 7/8n by 3/3 to obtain:
$\frac{4}{24n}$-($\frac{21}{24n}$)=$\frac{-17}{24n}$.
