6. A company produces laptops. Their products have 1 or 2 or 3 USB ports. The proportion of laptops based on the number of USB ports on them is given by the following pmf: p(k) = C (4 - k)3; k = 1,2,3 (k is the number of USB ports on the laptop) Note: p(1), p(2) and p(3) are the proportion of laptops with one, two and three USB ports, respectively. a. What is the value of the constant C? b. Let the random variable X be the number of USB ports on a randomly selected laptop. What is the mean of X? c. What is the variance of X