Do you think you have enough information to answer the question?

Why don't you draw a distribution that fits the question and see if you can calculate the fraction of 3 points/game.

I guess there is always the method of assuming something (something that the problem forgot to tell you to assume) and then answering the question based on the added assumption.

only discrete values of x are possible, and the number of possible x values is finite since it is not possible to score an infinite number of points in a game.

so mu = sum(xP(x)) if you know the probability distribution then you could work backward.

i also think you need more information about the situation.

It is also possible that the instructor wishes to demonstrate the fact that for a continuous random variable the probability of one specific outcome is zero. if that is the case then its a bad example problem.