Answer
$$13$$
Work Step by Step
$$\vec{AB}= \lt 2,0 \gt -\lt -1,2 \gt =\lt 3, -2 \gt \\ \vec{AC}=\lt 7,1 \gt -\lt -1,2 \gt =\lt 8,-1 \gt$$
Now, $$\vec{AB}\times\vec{AC}=\begin{vmatrix}\hat{i}&\hat{j}&\hat{k}\\3&-2&0\\8&-1&0\end{vmatrix}=(-3+16)\hat{k}= 13\hat{k}$$
and $$|\vec{AB}\times\vec{AC}|=\sqrt {(13)^2}=\sqrt {169}=13$$
