So I was working on a problem where I am provided with a PMF $p_X(k)= c/3^k$ for $k=1,2,3....$ I was able to calculate $c$ using the basic property of PMF and it came to be 2. I am not able to solve the next part which states that "Find $P(X\ge k)$ for all $k=1,2,3......$.
P.S :Here is the actual question:
Let X be a discrete random variable with probability mass function $p_X(k) = c/3^k$ for k = 1, 2, ... for some $c > 0$. Find $c$. Find $P(X\ge k)$ for all $k = 1, 2,3....$