Let A(-2, -1), B(1, 0) and C(4, 3) and D(x,y) be the vertices of a parallelogram ABCD taken in order.

Since the diagonals of a parallelogram bisect each other.

Therefoer, Coordinates of the mid - point of AC = Coordinates of the mid-point of BD.

Hence, fourth vertex of the parallelogram is (1, 2)