We can use this to transform the integral over the region $R$ to an integral over a new region $G$ in the $uv$-plane. The Jacobian determinant of this transformation is $|J| = 2$, so the integral becomes
$$\iint_{G} 2(x-y) |J| du dv = \iint_{G} 2(u+v - (u-v)) 2 du
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