$\displaystyle x* \sum_{1 }^{ \infty } na_nx^{n-1} +\sum_{0 }^{ \infty }a_nx^n$

I have to rewrite this expression with one just involving $\displaystyle x^n$ apparently the answer is $\displaystyle \sum_{0 }^{ \infty }(n+1)a_nx^n$ but i dont know how to combine the two series.. any help?