Let the three vertices of parallelogram ABCD be A(1,-1),B(-2,0) and C(1,2)
Let the fourth vertex be D(a,b)
By joining AC and BD intersect at point O
i.e, diagonals of parallelogram bisect each other.
Mid point of AC= ( (1+1)/2, (-1+2)/2 ) = (1,1/2)
Mid point of BD=( (x-2)/2, (y+0)/2 )
So, (x-2)/2 = 1 => x=4
y/2 = 1/2 => y=1
∴x=4,y=1
∴ The coordinate of fourth vertex =(4,1) ☺