## Calculus (3rd Edition)

$36$.
By using Theorem 1, we have $$\operatorname{Area}(G(\mathcal{D}))=|\operatorname{Jac}(G)| \operatorname{Area}(\mathcal{D})$$ And thus $$\operatorname{Area}(G(\mathcal{R}))=|\operatorname{Jac}(G)| \operatorname{Area}(\mathcal{R})=4\times 9=36.$$