Answer
$$M_{A B}=136 \mathrm{~N} \cdot \mathrm{m}
$$
Work Step by Step
$$
\begin{aligned}
& \mathbf{u}_{A B}=\frac{\{3.5 \mathbf{i}+0.5 \mathbf{j}\}}{\sqrt{(3.5)^2+(0.5)^2}} \\
& \mathbf{u}_{A B}=\{0.9899 \mathbf{i}+0.1414 \mathbf{j}\} \\
& M_{A B}=\mathbf{u}_{A B} \cdot\left(\mathbf{r}_{A D} \times \mathbf{F}\right)=\left|\begin{array}{ccc}
0.9899 & 0.1414 & 0 \\
2.5 & 0 & 4 \\
50 & -20 & -80
\end{array}\right| \\
& M_{A B}=136 \mathrm{~N} \cdot \mathrm{m}
\end{aligned}
$$
