Below are videos with the formula for finding the midpoint of a line segment if you know the ordered pairs of its endpoints. Also, a proof of the formula using geometry is provided in Part 1.
MF1: Midpoint Formula Part 1: http://youtu.be/qci-8fyl0ko
MF2: Midpoint Formula Part 2: http://youtu.be/GCD5Edum9ZM
File name: midpt1.mp4, etc.