oscillations on a string
two fixed ends (node) $L$ units apart
# 1st harmonic
$\lambda=2L$
$$f=\frac{c}{\lambda}$$
$f=\frac{c}{2L}$
# 1st overtone (2nd harmonic)
$\lambda=L$
$f=\frac{c}{L}$
# 2nd overtone (3rd harmonic)
$\lambda=\frac{2L}{3}$
$f=\frac{3c}{2L}$
General equations for the nth harmonic
$$\lambda=\frac{2L}{n}$$
$$f=\frac{nc}{2L}$$