In this problem we use the change of variables x=2s+t, y=s−3t to compute the integral ∫R(x+y)dA, where R is the parallelogram with vertices (x, y)=(0,0), (4,2), (7,−7), and (3,−9). First find the magnitude of the Jacobian, ∣∣∂(x, y)/∂(s, t)∣∣ then, ∫R(x+y)dA=?