Problem $1$

A string of length L is tied at both ends, and a harmonic mode is created with a frequency of $49\mathrm{.}0$ Hz $.$ If the next successive harmonic is at $56\mathrm{.}0$ Hz and the speed of transverse waves on the string is $63\mathrm{.}0$m$/$s$,$ find the length of the string. Note that the fundamental frequency is not necessarily $49\mathrm{.}0$ Hz $.$