Answer
77.89 m/s
Work Step by Step
Here we use the equation $V=\sqrt {\frac{F}{\mu}}$ where, V - speed of the wave, F - tension force, $\mu$ - mass per unit length
The maximum speed occurs when the F becomes maximum.
$V_{max}=\sqrt {\frac{F_{max}}{\mu}}$ ; Let's plug known values into this equation.
$V_{max}=\sqrt {\frac{415kgm/s^{2}}{68.4\times10^{-3}kg/m}}=77.89 m/s$