Answer
42
Work Step by Step
$\vec{AB}= 16\hat{i}-5\hat{j}$
$\vec{AC}= 4\hat{i}+ 4\hat{j}$.
$\vec{AB}\times\vec{AC}=\begin{vmatrix}\hat{i}&\hat{j}&\hat{k}\\16&-5&0\\4&4&0\end{vmatrix}$$=(64+20)\hat{k}= 84\hat{k}$.
$|\vec{AB}\times\vec{AC}|=\sqrt {84^{2}}= 84$.
Area of the triangle = $\frac{1}{2}|\vec{AB}\times\vec{AC}|=\frac{1}{2}\times84=42$
