Given:
The three vertices of the parallelogram are (-3,9), (0,-3), (6,-6).
To find:
The fourth vertex of the parallelogram.
Solution:
Consider the three vertices of the parallelogram are A(-3,9), B(0,-3), C(6,-6).
Let D(a,b) be the fourth vertex.
Midpoint formula:

We know that the diagonals of a parallelogram bisect each other. So, the midpoints of both diagonals are same.
Midpoint of AC = Midpoint BD


On comparing both sides, we get


And,




Therefore, the fourth vertex of the parallelogram is (3,6).