Answer:
The answer is 7.5 feet.
Step-by-step explanation:
When a wave has a height of 5 feet and a wavelength of 15 feet then it has a wave base of 7.5 feet. Wave base is equal to the half of the wavelength so if you consider your wavelength to be 15 feet then it is divided into 2 that would give you answer of 7.5 feet.
I hope I helped, please correct me if I'm wrong!