Consider the triangle ABC. Let P divide the segment AB in the ratio s : t, and Q divide the segment BC in the ratio t : s.

(a) Draw a sketch of the situation.

(b) Express the vector PQ in terms of t, s, and AC.

(c) Hence explain why PQ and AC are parallel

I understand Part A and realise that the length of AB = PQ, and can also see how PQ is parallel to that of AC. Just unsure on how to express PQ.