While standing at the edge of the roof of a building, a man throws a stone upward with an initial speed of 6.45 m/s. The stone subsequently falls to the ground, which is 13.5 m below the point where the stone leaves his hand. Ignore air resistance and use g = 9.81 m/s² for the acceleration due to gravity. 

a. At what speed v does the stone impact the ground? 

b. How much time ¢ does the stone spend in the air?