*5. A chord joins any two points A and B on the parabola whose equation is y^2 = 4x. If C is the midpoint of AB, and CD is drawn parallel to the x-axis to meet the parabola at D, prove that the tangent at D is parallel to chord AB. *

I've attempted to graph the situation (attempted graph is attached), however I have no idea how to represent it with equations. What are my constraints? What maximum/minimum am I trying to solve for?

I've understood this unit up to now, though I simply can't find an answer to this question. Help would be strongly appreciated.