Construct OP and OQ. Since midpoint of AB is P, then the line OP must be perpendicular to the chord (radii bisecting a chord is at a right angle, or a theorem like that). Therefore, angle APO = 90. This is similar with angle AQO. Since Q is midpoint of AC then OQ is perpendicular. Since angle AQO = 90 and angle APO = 90, angle APO + angle AQO = 90+90= 180. Since opposite angles of a quadrilateral are supplementary then the quadrilateral lies on a circle and is therefore concyclic.