$$\ r_{BC} = ( 15 - 3 )i + ( 10 - 2 )j + ( 0 - 0 )k $$$$ r_{BC} = \ +0i +0j +0k \ $$
$$ F = 840 \left[ \frac{+0\mathbf{i} + +0\mathbf{j} - +0\mathbf{k}}{\sqrt{(+0)^2 + (+0)^2 + (+0)^2}} \right] = +0\mathbf{i} +0\mathbf{j} +0\mathbf{k} \ \text{N} $$$$ A_x = +0 \, \text{N} $$$$ A_y = +0 \, \text{N} $$$$ A_z = +0 \, \text{N} $$
$$\ r_{ac} = ( 15 - 0 )i + ( 10 - 0 )j + ( 0 - 0 )k $$$$ r_{ac} = \ +0i +0j \ $$
\[
\mathbf{M}_A + \begin{vmatrix}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
15 & 10 & 0 \\
+0 & +0 & +0
\end{vmatrix} = 0
\]
\[
\mathbf{M}_A +0\mathbf{i} +0\mathbf{j} = 0
\]
$$ M_{Ax} = 0 \, \text{kN-m} $$
$$ M_{Ay} = 0 \, \text{kN-m} $$
$$ M_{Az} = 0 $$