Hello, I am reviewing math over the summer and am having trouble with this problem:The trouble is that this isn't from a geometry textbook, it's from the precal review section of a calculus textbook. The section it appears in covers only the Pythagorean theorem, the distance formula, and the midpoint formula. Hence I assume I'm supposed to prove this algebraically, with not much more information than that. How would I do this?

Prove that the line segments joining the midpoints of the opposite sides of a quadrilateral bisect each other.