Answer
$$
M_A=\{-175 \mathbf{i}+5600 \mathbf{j}-900 \mathbf{k}\} \mathrm{lb} \cdot \mathrm{ft}
$$
Work Step by Step
The coordinates of points $A$ and $B$ are $A(0,0,1.5) \mathrm{ft}$ and $B(8,0.25,1.5) \mathrm{ft}$, respectively.
$$
\begin{aligned}
\mathbf{r}_{A B} & =(8-0) \mathbf{i}+(0.25-0) \mathbf{j}+(1.5-1.5) \mathbf{k} \\
& =\{8 \mathbf{i}+0.25 \mathbf{j}\} \mathrm{ft}
\end{aligned}
$$
Moment of $\boldsymbol{F}$ About Point $\boldsymbol{A}$.
$$
\begin{aligned}
M_A & =\mathbf{r}_{A B} \times \mathbf{F} \\
& =\left|\begin{array}{ccc}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
8 & 0.25 & 0 \\
400 & -100 & -700
\end{array}\right| \\
& =\{-175 \mathbf{i}+5600 \mathbf{j}-900 \mathbf{k}\} \mathrm{lb} \cdot \mathrm{ft}
\end{aligned}
$$
