Answer
$$dv = {e^{ax}}dx$$
Work Step by Step
$$\eqalign{
& \int {{x^n}{e^{ax}}dx} \cr
& {\text{We can choose }}{x^n}{\text{ as }}u,{\text{ because by the power rule}} \cr
& du = n{x^{n - 1}}{\text{ the exponent will be reducing, and then we}} \cr
& {\text{need to choose }}{e^{ax}}dx{\text{ as }}dv,{\text{ besides it is easily to integrate}}{\text{.}} \cr
& u = {x^n} \cr
& dv = {e^{ax}}dx \cr} $$