\displaystyle \sum_{n=k}^{\infty}pq^n = p\sum_{n=k}^{\infty}q^n = p(\sum_{n=0}^{\infty}q^n - (1+q+q^2+...+q^{k-1})) = p(\frac{1}{1-q} - \frac{1-q^k}{1-q}) =