UPSC CDS Math Lines and Angles Questions

Three parallel lines x,y and z are cut by two transversals m and n. Transversal m cuts the lines x,y, at P, Q, R respectively; and Transversal n cuts the lines x, y, z at L > M, N respectively. If PQ = 3 cm, QR = 9 cm and MN= 10.5 cm, then what is the length of LM ?