Because the crests occur every 5 seconds, the period is
T = 5 s.
By definition, the frequency is
f = 1/T = 1/5 = 0.2 Hz
The distance between crests is 15 m, therefore the wavelength is
λ = 15 m
By definition, the velocity, v, is
v = fλ
= (0.2 1/s)*(15 m)
= 3 m/s
Answer: 3 m/s
