Let three vertices be A(-2, -1), B(1, 0) and C(4, 3) and fourth vertex be D(x, y)

It is given that quadrilateral joining these four vertices is parallelogram.

∴□ABCD is parallelogram

We know that diagonals of parallelogram bisect each other, ie midpoint of the diagonals coincide.

Let E(x_m , y_m) be the midpoint of diagonals AC and BD.

By midpoint formula,

x = , y =

For diagonal AC,

x_m = , y_m =

∴ x_m = , y_m =

∴ E(x_m , y_m) ≡ (1, 1)

For diagonal BD,

1 = , 1 =

∴ x = 2 – 1 , y = 2 – 0

∴ x = 1 and y = 2

Hence, our fourth vertex is D(1, 2)