Simplify -3/(4n^3)+(13n-4)/(4n^3)

$\frac{- 3}{4 n^{3}} + \frac{13 n - 4}{4 n^{3}}$

Combine the numerators over the common denominator.

$\frac{- 3 + 13 n - 4}{4 n^{3}}$