Create a function that takes a number of a guitar string and the number of the fret and returns the corresponding frequency of the note.
String Frequency * 2**(fret/12).So, one fret = a semitone = a half step. From D to D♯ for instance.
fret_freq(5, 12) ➞ 220
fret_freq(4, 6) ➞ 207.65
fret_freq(2, 23) ➞ 932.32
N/A