Answer
$f_1=131Hz$
G4, C5, E5, and G5 are the 3rd, 4th, 5th, and 6th harmonics respectively.
Work Step by Step
$fn=392Hz$
$f(n+1)=523Hz$
$f(n+2)=659Hz$
$f(n+3)=784Hz$
By taking the average of the differences, the answer is approximately 131Hz
From $f_1=\frac{v}{2l}$, we know $l=1.31m$
Therefore, G4, C5, E5, and G5 are the 3rd, 4th, 5th, and 6th harmonics respectively.
