If ΔPQR and ΔLMN are similar and 3PQ = LM and MN = 9 cm, then QR is equal to:

$$\eqalign{ & \frac{{PQ}}{{LM}} = \frac{{QR}}{{MN}} = \frac{{PR}}{{LN}} \cr & \left( {\vartriangle PQR\,{\text{and}}\,\vartriangle LMN\,{\text{are}}\,{\text{similar}}} \right) \cr & \frac{{PQ}}{{LM}} = \frac{{QR}}{{MN}} \cr & \frac{1}{3} = \frac{{QR}}{9} \cr & QR = 3\,{\text{cm}} \cr} $$