How do I approach this question?

8. [H] Let O, A, B, C be points in a plane. Suppose that X is the midpoint of BC, Y is the point on AC with AY : YC = 3 : 1, and Z is the point on AB with AZ : ZB = 3 : 1. Let OA = a, OB = b, and OC = c. a) Write down the vectors OX, OY and OZ in terms of a, b, c. b) Let T be the point on AX with AT : TX =6 : 1. Write OF in terms of a, b, c. c) Show that T lies on both CZ and Y B