Answer
$1.21m$
Work Step by Step
We know that
$f=\frac{v}{2L}$
$\implies f=2f_1L_1=2f_2L_2$
Now we can find the length of the string
$L_2=\frac{f_1}{f_2}L_1$
We also know that $f_2=f_1+f_{beat}$
$\implies L_2=\frac{f_1}{f_1+f_{beat}}L_1$
We plug in the known values to obtain:
$L_2=(\frac{130.9}{130.9+4.33})(1.25)=1.21m$
