is this the right method of proving:
\begin{align} Cov(a_0+a_1R_1+a_2R_2, Q_1)=a_1Cov(R_1,Q_1)+a_2(R_2,Q_1) \end{align}
By
\begin{align} =Cov(a_0+a_1R_1+a_2R_2, Q_1) \end{align}
\begin{align} =Cov(a_1R_1+a_2R_2, Q_1) \end{align}
\begin{align} =E[((a_1R_1+a_2R_2)-E(a_1R_1+a_2R_2))(Q-EQ_1)] \end{align}
\begin{align} =a_1Cov(R_1,Q_1)+a_2(R_2,Q_1) \end{align}
Since
\begin{align} E[(X-EX)(Y-EY)] = Cov(X,Y) \end{align}