Take into account that the relation in betwee the fundamental and the harmonics is given by:
f = fo·n
where f is the frecuency of the nth harmonic and fo is the fundamental.
Then, for the 8th harmonic (n = 8) and a fundamental of 53 Hz, you obtain for f:
f = (53 Hz)(8)
f = 424 Hz
Hence, the frequency of the 8th harmonic is 424 Hz
