If the fundamental frequency of a musical instrument is 42Hz, what is the frequency of the second harmonic?

For the majority of instruments f = n f0 where f is the resonating frequency, n is any whole number and f0 is the fundamental.

This applies to trumpets, violins, flutes and a broad range.

In such a case the first harmonic would be at n=1 and the second harmonic would be at n=2

which gives a frequency of 84 Hz