Find unknown entries in a matrix equation
The matrix equation below is true
\[\begin{bmatrix} 9 & 9 \\ 1 & 2 \end{bmatrix} + \begin{bmatrix} 7 & 12 \\ 5 & x \end{bmatrix} = \begin{bmatrix} 16 & 21 \\ 6 & 6 \end{bmatrix}\]
The matrix equation below is true