To answer this question, we need to first start with the formula for the CAPM:
E[Ri] = Rf + ßi[E[Rm - Rf]] + αi where...
Rf = Risk-free Rate
E[Ri] = Expected Return of Asset i
ßi = Asset's Beta, which is the part of the asset's excess return explained by systematic risk.
E[Rm - Rf] = Market Risk Premium
αi = Asset's alpha, which is the part of the asset's excess return NOT explained by systematic risk.
The excess return can be characterized by subtracting Rf from both sides of the CAPM formula:
E[Ri] - Rf = αi + ßi[E[Rm - Rf]]
Remember, if the CAPM is correct, αi should be zero. From here, we can plug in the values given in the problem:
E[Ri] - Rf = 2.3 + 3.1[12] = 39.5%
Thus, the excess return is 39.5%.