Calculate the Riemann sum for $f(x,y)=3 x - 5 y$ and domain $\mathcal{D}$ in Figure 2 with two choices of sample points, $\bullet$ and $\circ$.
Which do you think is a better approximation to the integral of $f$ over $\mathcal{D}$ ? Why?

$S_{3,4}(\bullet) =$
$S_{3,4}(\circ) =$