Answer:
The coordinates of point C are (b,a).
Explanation:
Three vertices of rectangle ABCD are A(0, 0), B(0, a), and D(b, 0).
Let the coordinates of point C are (x,y)
We know that the diagonals of a rectangle bisect each other. It means the midpoint of both the diagonals are same.
Midpoint of diagonal AC is

Midpoint of diagonal BD is

Equating the midpoint we get

On comparing both the sides, we get

Therefore the coordinates of point C are (b,a).