A substitution that will simplify this integrand $$\iint_{R}^{} x \sin(6x + 7y) - 3y \sin(6x + 7y) dA$$
So I chose $u = 3y$ and $v = 6x + 7y$. So then $x$ will be replaced with $\frac{3u - 7v}{18}$. It seems correct, but does this truly "simplify" the integrand? Or have I done something incorrectly?

Hint: Choose $u=6x+7y$ and $v=x-3y$ to get 
$$\int\int vsin(u) (\frac{1}{25})dudv$$
where $\dfrac{1}{25}=\dfrac{\partial(x,y)}{\partial(u,v)}$
