Answer to the summation

Question

I have this summation Check the image

How did I get the RHS of answer?

Answer 1

Hint: if $|q|<1$ then $\displaystyle\sum_{k=0}^{\infty}aq^{k-1}=\frac{a}{1-q}.$ Obesrve that the 2nd summation starts from $m=1$, so adapt this formula.

Answer 2

For a real number $q$ with $|q|<1$ we have

$ \sum_{n=0}^{\infty}q^n=\frac{1}{1-q}$ and $ \sum_{n=1}^{\infty}q^n=\frac{q}{1-q}$.