c
Let third vertex $=(\mathrm{h}, \mathrm{k})$
lies on $y=k+3$
$\mathrm{k}=\mathrm{h}+3$ .........$(1)$
$D = \frac{1}{2}\left| {\begin{array}{*{20}{c}}
h&k&1\\
2&1&1\\
3&{ - 2}&1
\end{array}} \right| = \pm 5$
$\mathrm{h}(1+2)-\mathrm{k}(2-3)+1(-4-3)=\pm 10$
$3 h+k-7=\pm 10$
$3 \mathrm{h}+\mathrm{k}=17$ .........$(2)$
$3 h+k=-3$ .........$(3)$
from $(1)$ and $(3)$
$\mathrm{h}=-\frac{3}{2}, \mathrm{k}=\frac{3}{2}$