7. (5% each, 15% in total) Consider a random variable $X$ with its PMF

$$P_X(x) = \begin{cases} \frac{c}{x} & \text{if } x = 1, 2, 3; \\ 0 & \text{else,} \end{cases}$$

for some constant $c$.

(a) What is the constant \( c \)?