Estimable functions are the combinations of $\theta_2-\theta_1$ and $\theta_2+\theta_3$

$Y_1$, $Y_2$ and $Y_3$ are three uncorrelated random variables with common variance $\sigma^2$. Further $E(Y_1)=\theta_2-\theta_1$, $E(Y_2)=E(Y_3)=\theta_2+\theta_3$. Then: