Simplify (-5x^3)/(5x+8)+(6x^3)/(5x+8)

$\frac{- 5 x^{3}}{5 x + 8} + \frac{6 x^{3}}{5 x + 8}$

Combine the numerators over the common denominator.

$\frac{- 5 x^{3} + 6 x^{3}}{5 x + 8}$