Question

if waves on the lake have a frequency of 3 hertz and a wavelength of 4 meters,what is the speed of the waves?

Answer 1

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.