Blending Two Weight Matrices

Two weight matrices from a small model:

$A = \begin{bmatrix} 6 & 4 \\ 9 & -2 \end{bmatrix}, \qquad B = \begin{bmatrix} 3 & -5 \\ -3 & 7 \end{bmatrix}$

You blend them into a single matrix $W = A + cB$, and you pick the scalar $c$ so that the weight in position $(2,1)$ is switched off completely — that is, $W_{2,1} = 0$.

For that value of $c$, what is $W_{1,2}$?