Answer
$v = 40~m/s$
Work Step by Step
We can see that this is the third harmonic since there are 3 antinodes. We can find the wavelength.
$\lambda = \frac{2L}{n}$
$\lambda = \frac{(2)(0.60~m)}{3}$
$\lambda = 0.40~m$
We can find the wave speed.
$v = f~\lambda$
$v = (100~Hz)(0.40~m)$
$v = 40~m/s$
