Work, Power and Enerhgy: Wind Turbines

Could you please explain what formula and step are involved and please provide (example) problem if possible?

a)Assume a wind turbine with a hub 50 meters above the ground, a rotor diameter of 75 meters and a wind-conversion efficiency of 25 percent. The turbine operates in an area with an average wind-power density of 650 watts/sq meter at 50 meters altitude. How much electricity (kWh) can the turbine generate per year?

b) Wind densities greater than or equal to 500 watts/sq meter at an altitude of 50 meters are present on 1.500 percent of the land area of the US. If, on average, wind farms contain 8 turbines/sq km, how much electricity (kWh/yr) could be generated from these wind turbines? (The area of the US is 7,827, 989 sq km).