A copper rod is given a sharp compressional blow at one end. The sound of the blow, traveling through air at -2.79 degrees Celsius, reaches the opposite end of the rod 9.39ms later than the sound transmitted through the rod.

What is the length of the rod? The speed of sound in copper is 3550m/s and the speed of sound in air at -2.79 degrees celsius is 331 m/s. Answer in units of m
I did:
T-t = 9.39
x((1/331)-(1/3550))=9.39 and solved for x but it was incorrect.