b. Use the change of variables \( u=3 x+y, v=x+3 y \) to compute the integral
\[
\iint_{R}(x+3 y) d A
\]
where \( R \) is the parallelogram enclosed by the lines
\[
3 x+y=9,3 x+y=33, x+3 y=3, x+3 y=27
\]
[Its vertices are \( (0,9),(9,6),(12,-3) \) and \( (3,0) \).
(10 marks)