Regression: One may argue that the expected return on asset $i,x_1,$ is the sum of the return on the risk$-$free asset plus a risk premium. The risk premium on ${}_i$ must be proportional to the expected excess return on the portfolio of all risky assets, $x_{m_1}$ over the risk$-$free asset.

$E(Ri)=Rf+[E(Rm)-Rf]i$

How would you specify $i?$