Hello student, the mean of X is also the expected value of X, also known as E(X).
Given the E(X) value, you first create an equation that is equal to E(X).

Next, Do note that the probability of all possible outcomes is 1. Thus you can sum up the row of P(X=x) be equals to 1.

After which, you can then solve simultaneous equations for both the equations to get the values of a and b.

Hope that helps!!