Let the coordinates of the fourth vertex D be (x, y).
We know that diagonals of a parallelogram bisect each other.
∴ Midpoint of AC = Midpoint of BD …(i)
Coordinates of the midpoint of AC are
⇒ 3 + x = 4 and 7 + y = 6
⇒ x = 1 and y = -1
Thus, the coordinates of the vertex D are (1, -1)