You have to use the equation v=λf v is the velocity of the wave λ is the wave length of the wave (how far the wave goes in one cycle) f is the frequency of the wave (how long one cycle is)
V=4m×3Hz v=12m/s
Hz stands for s⁻¹ which is why the final units is m/s
I hope this helps. Let me know if anything is unclear.