Quadrilateral ABCD has the following vertices: A(0,6)
B(3,5)
C(0, -4)
D(-3,-3)
Also, angle a is a right angle.
Is quadrilateral ABCD a rectangle, and why?

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  yes

Step-by-step explanation:

The figure can be shown to be a parallelogram by showing the sum of endpoints of the diagonals is the same.

  A +C = B +D

  (0, 6) +(0, -4) = (0, 2) = (3, 5) +(-3, -3) . . . . diagonals bisect each other

If the diagonals of a quadrilateral bisect each other, it is a parallelogram. A parallelogram with a right angle is a rectangle. So, ABCD is a rectangle.

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Additional comment

The midpoint of each diagonal is half the sum of the end point coordinates. That is, the midpoints are (0, 2)/2 = (0, 1). Since calculation of the midpoints requires both sums be divided by 2, we can tell the midpoints are the same if the sums are the same.

true? is that what you're asking or do you mean something else?