Answer
(a) See the image below.
(b)
$M_{AC}=(2.5, 3)$
$M_{DB}=(2.5, 3)$
(c) As we can see the midpoints are the same. So with this point both of the diagonals are divided into halves.
So, they bisect each other.
Work Step by Step
(a) We simply plot the points and connect them. See the image above.
(b) We have diagonals $AC$ and $DB$. Let $M_{AC}$ and $M_{DB}$ be their midpoints.
$M_{AC}=(\frac{-2+7}{2}, \frac{-1+7}{2})=(2.5, 3)$
$M_{DB}=(\frac{1+4}{2}, \frac{4+2}{2})=(2.5, 3)$
(c) As we can see the midpoints are the same. So with this point both of the diagonals are divided into halves.
So, they bisect each other.