For this type of questions of solving 2 unknowns, we need to form 2 equations. We can form the first one via the Expectation formula of a pdf function (the question gave us the expectation value too). We can form another formula with another property of pdf functions too which is the area under a pdf function is equal to 1. (Probability of all cases sum up to 1). With the 2 equations, we can solve it simultaneously, hope this helps ;)