Since the region is defined in polar coordinates, we convert the integral to polar coordinates. We have $x = r \cos \theta$ and $dA = r \, dr \, d\theta$. Thus, the integral becomes
$$\iint_R 3x \, dA = \int_{\pi}^{\frac{3}{2}\pi} \int_3^6 3(r \cos \theta) r \, dr
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