Conditional variance from a block-diagonal covariance

Suppose $\underline X_{3\times1}\sim\text{MNormal}_3\!\left( \underline0,\left(\begin{smallmatrix}1&0.5&0\\0.5&4&0\\0&0&1 \end{smallmatrix}\right)\right)$. Hence $\var(2X_2\mid X_1=x)$ is equal to: