How to find an appropriate change of variables in double integrals, Calculus 3, Homework 15.9B Help

Change of variables in a double integral. Evaluate the integral by using an appropriate transformation and Jacobian.
Remarks on
#2 Use the given transformation to evaluate the integral...where R is the region in the first quadrant bounded by the lines ...
#4 Evaluate the integral by making an appropriate change of variables...where R is the trapezoidal region with vertices (7, 0), (8, 0), (0, 8), and (0, 7)
#5 Evaluate the integral by making an appropriate change of variables....where R is given by the inequality
8|x| + 8|y| ≤ 8