We have that OP = 2/3 OA and suppose that PQ = k PC.
Then OQ = OP + PQ = 2/3 OA + k PC = 2/3 OA + k (OC - OP) = 2/3 OA + k (OC - 2/3 OA).
But OQ =lambda OB = lambda (OA + OC).
So you can now equate the coefficients of OA and OC in :
lambda (OA + OC) = 2/3 OA + k (OC - 2/3 OA).
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