Answer
$a.\displaystyle \quad D=\frac{429}{p}.$
$b.\quad 143$ bags sold.
Work Step by Step
If the demand $D$ is inversely related to the price $p$, then
there is a nonzero constant $k$ such that
$D=\displaystyle \frac{k}{p}$
a.
If, when p=2.75, D=156,
substituting, we find k:
$156=\displaystyle \frac{k}{2.75}\quad/\times 2.75$
$k=429$
Thus, we write: $\displaystyle \quad D=\frac{429}{p}.$
b.
If p=3,
$D=\displaystyle \frac{429}{3}$ = $143$ bags sold.