Answer
$157.5\;m/s$
Work Step by Step
The lowest resonant frequency, i.e., the first harmonic of the stretched string is $105\;Hz$. Thus, we have
$f_1=\frac{v}{2L}$
or, $v=2f_1L$
Substituting the given values, we obtain
$v=2\times105\times0.75\;m/s$
or, $v=157.5\;m/s$
Therefore, the wave speed is $157.5\;m/s$
