Thomas' Calculus 13th Edition

$$42$$
Area of the triangle is: $Area =\dfrac{1}{2}|\vec{AB}\times\vec{AC}|$ $$\vec{AB}= 16\hat{i}-5\hat{j} \\ \vec{AC}= 4\hat{i}+ 4\hat{j}$$ Now, $$\vec{AB}\times\vec{AC}=\begin{vmatrix}\hat{i}&\hat{j}&\hat{k}\\16&-5&0\\4&4&0\end{vmatrix} \\=(64+20)\hat{k} \\= 84\hat{k}$$ and $$|\vec{AB}\times\vec{AC}|=\sqrt {84^{2}}= 84$$ Now, $Area=42$