$\displaystyle \iint_s (x+y) d\sigma$, where S is the portion of the plane $\displaystyle x + 2y - 3z = 4$ within $\displaystyle 0 \leqslant x \leqslant 1, 1 \leqslant y \leqslant 2$

You can't convert to polar co-ordinates as its not a circle, right?