Answer
30.426 m/s
Work Step by Step
Here we use the equation $V=\sqrt {\frac{F}{\mu}}$ where, V - velocity of the wave, F - Tension of the string, $\mu$ - mass per unit length.
$V=\sqrt {\frac{F}{\mu}}$ ; Let's apply V = 18 m/s, F = 14 N into this equation.
$18\space m/s=\sqrt {\frac{14kgm/s^{2}}{\mu}}$
$324\space m^{2}/s^{2}=\frac{14\space kgm/s^{2}}{\mu}=\gt \mu=\frac{7}{162}kg/m$
When F = 40 N, we can write,
$V=\sqrt {\frac{40\space kgm/s^{2}}{7/162kg/m}}=30.426\space m/s$
