Given that the ${A}_{11}$ and ${A}_{22}\in {\mathbb{R}}^{3x3}$ are invertible, ${A}_{21}\in {\mathbb{R}}^{3x3}$, and ${b}_{1},{b}_{2},{x}_{1},{x}_{2}\in {\mathbb{R}}^{3}$, then solve for ${x}_{1}$ and${x}_{2}$ from

$\left[\begin{array}{cc}{A}_{11}& 0\\ {A}_{21}& {A}_{22}\end{array}\right]$ $\begin{array}{r}\left[\begin{array}{c}{x}_{1}\\ {x}_{2}\end{array}\right]\end{array}=$ $\begin{array}{r}\left[\begin{array}{c}{b}_{1}\\ {b}_{2}\end{array}\right]\end{array}$

What are ${x}_{1}$ and ${x}_{2}$ in terms of ${A}_{11},{A}_{21},{A}_{22},{b}_{1},{b}_{2}$?

$\left[\begin{array}{cc}{A}_{11}& 0\\ {A}_{21}& {A}_{22}\end{array}\right]$ $\begin{array}{r}\left[\begin{array}{c}{x}_{1}\\ {x}_{2}\end{array}\right]\end{array}=$ $\begin{array}{r}\left[\begin{array}{c}{b}_{1}\\ {b}_{2}\end{array}\right]\end{array}$

What are ${x}_{1}$ and ${x}_{2}$ in terms of ${A}_{11},{A}_{21},{A}_{22},{b}_{1},{b}_{2}$?