Answer
$V=\sqrt {yg}$
Work Step by Step
Please see the attached image first.
Here we use the equation $V=\sqrt {\frac{F}{\mu}}$ where, V - speed of the wave, F - tension force, $\mu$ - mass per unit length
$V=\sqrt {\frac{F}{\mu}}$ ; Let's plug known values into this equation.
$V=\sqrt {\frac{T}{\mu}}=\sqrt {\frac{mg}{m/y}}=\sqrt {yg}$
