$ \bar{F}=80\hat{i}+50\hat{j}-60\hat{k}$ passes through point A (6, 2, 6) $\therefore{}\ {\bar{M}}_B=\left\vert{}\begin{array}{ ccc} \hat{i} & \hat{j} & \hat{k} \ (6-8) & (2-1) & (6-4) \ 80 & 50 & -60 \end{array}\right\vert{}$ $\therefore{}\ {\bar{M}}_B=\left\vert{}\begin{array}{ ccc} \hat{i} & \hat{j} & \hat{k} \ -2 & 1 & 2 \ 80 & 50 & -60 \end{array}\right\vert{}$ $=\hat{i}\left(-60-100\right)-\hat{j}\left(120-160\right)+\hat{k}(-100-80)$ ${\bar{M}}_B=-160\hat{i}+40\hat{j}-180\hat{k}$