Let X follows Poisson distribution with parameter $\lambda\gt0$ and $Y=aX$, where $a\gt0$ is a constant.
Q. What will be the $PMF$ of $Y$?
Since by using the M.G.F we have $M_Y(t)=M_X(at)=(e^{\lambda(e^{at}-1)})$. What next I should do to derive its distribution.