We have $$\mathbf{B} =\left( \begin{array}{ccc} a_{11} & a_{12}c -a_{13}s& a_{12}s+a_{13}c \\ a_{12}c -a_{13}s & a_{22}c^2+a_{33}s^2 -2a_{23}sc& (a_{22}-a_{33})sc +a_{23}(c^2-s^2) \\ a_{12}s+a_{13}c & (a_{22}-a_{33})sc +a_{23}(c^2-s^2) & a_{22}s^2+a_{33}c^2 +2a_{23}sc \end{array} \right).$$ or $$\begin{align*} b_{11} =& a_{11} \\ b_{12} =& a_{12}\cos\theta - a_{13}\sin\theta , 1 \ne 2, 1 \ne 3 \\ b_{13} =& a_{13}\cos\theta + a_{12}\sin\theta , 1 \ne 2, 1 \ne 3 \nonumber\\ b_{22} =& a_{22}\cos^2\theta - 2a_{23}\cos\theta \sin\theta +a_{33}\sin^2\theta\nonumber\\ b_{33} =& a_{33}\cos^2\theta +2a_{23}\cos\theta \sin\theta +a_{22}\sin^2\theta\nonumber\\ b_{23} =& (a_{22}-a_{33})\cos\theta \sin\theta +a_{23}(\cos^2\theta-\sin^2\theta)\nonumber \end{align*}$$ We will fix the angle $\theta$ so that $b_{23}=0$.