For X which is a continuous random variable that is uniformly distributed and takes values between 3 and 9, the minimum value of the random variable X is 3 (it’s the value of a) and the maximum value is 9 (it’s the value of b). Then X~U (a=3, b=9) and the probability density function of the continuous random variable X is defined as, f (x) = 1/(b - a) = 1/(9 - 3)= 1/6 for 3?x?9. The mean is equal to (9 + 3)/2 = 6. The answer is E.