Answer:-(2a,a) coordinates he should assign to the fourth vertex of the parallelogram.
Explanation:-
Let ABCD be the vertices of the given parallelogram such that A=(0.,0) , B=(a,0) , C(x,y) and D=(a,a)
Construct the diagonals AC and BD of parallelogram ABCD .
We know that diagonals of a parallelogram bisect each other.
⇒ mid point of AC= mid point of BD...(1)
Mid point of BD
........(2)
Mid point of AC
......(3)
Substitute (2) and (3) in (1), we get


⇒C=(2a,a)
⇒ (2a,a) coordinates he should assign to the fourth vertex of the parallelogram.