$ \textbf{Answer:}$
$ -\frac{1}{x}- \frac{3}{x^2}+C$
$\textbf{Explaination:}$
$ \int \left( \frac{1}{x^2} + \frac{6}{x^3} \right) dx = \int \frac{1}{x^2} dx + \int \frac {6}{x^3}dx$ (sum rule of integrals)
$ = \int x^{-2} dx + 6 \int x^{-3} dx$ (writing the fractions in terms of the exponents and taking out constants out of the integral)
$ = \frac{1}{-2+1}x^{-2+1} + c_1 + \frac{6}{-3+1}x^{-3+1} + c_2$ (integration exponents rule)
$ = -\frac{1}{x} + c_1 -\frac{3}{x^2} + c_2$ (simplifying)
$ = -\frac{1}{x} -\frac{3}{x^2} +C $ ($ C = c_1 + c_2$ is a constant of integration)