The diagonals of a parallelogram bisect each other.
That means they have the same midpoint.
See the Math Notes box in Lesson 7.3.3 for help finding the midpoint of the line segment which connects B and D.
The midpoint is (6, 21).
Use the eTool below to explore the diagonals of a parallelogram.
Click the link at right for the full version of the eTool: GC 8-51 HW eTool