The three vertices of a parallelogram are (3, 4), (3, 8) and (9, 8). Find the fourth vertex.

Consider A(3, 4), B (3, 8) and C(9, 8).

Let the coordinates of fourth vertex are D (x, y)

In a parallelogram diagonals bisect each other

Coordinate of mid point of AC =

Therefore coordinates of mid point of AC are (6, 6)

Coordinate of mid point of BD =

Coordinates of point D are

Therefore coordinates of fourth vertex D are (9, 4)