In the triangle OAB, OA = a and OB = b

The point P is the point on AB such that AP = 2PB and Q is a point such that OP = 3QP

Okay, this is a long question, I just couldn't do the last part.

The point R is on OA so that RQ is parallel to AB. Show that OR = 2/3 a (Hint: Let QR = cBA, where c is a scalar quantity)

I figured that OR = OQ + QR, and

OQ = 2/9 a + 4/9 b

now I am stuck.... any help will be appreciated. Thanks