Let A (-1,4) and B(5,2) are the vertices of the triangle and let D(0,3) is the midpoint of side AC.
Let co-ordinate of C be (x,y).
Consider D(0,3) as midpoint of AC
By midpoint formula,
(-1+x)/2 = 0
-1 +x = 0
x = 1
By midpoint formula,
(4+y)/2 = 3
4+y = 6
y = 6-4 = 2
So the co-ordinates of C are (1,2).
Consider D(0,3) as midpoint of BC
By midpoint formula,
(5+x)/2 = 0
5 +x = 0
x = -5
By midpoint formula,
(2+y)/2 = 3
2+y = 6
y = 6-2 = 4
So the co-ordinates of C are (-5,4).
Hence the co-ordinates of the point C will be (1,2) or (-5,4).