$$\left[\begin{array}{c}{\mathrm{A.}}_{1.}\\ B_{1.}\end{array}\right]=\left[\begin{array}{cc}{T}_{11}& T_{12}\\ T_{21}& T_{22}\end{array}\right]\left[\begin{array}{c}{B}_{2.}\\ {\mathrm{A.}}_{2.}\end{array}\right],$$