Let t denote the time measured in seconds. is the gravitational acceleration(?).

The path of the stone is described by two independent movements: a horizontal and a vertical:

to a): Plug in t = 2 into both equations to get (20, 180.38)

to b): The stone reaches the ground when y(t) = 0. Solve for t. (I've got t = 6.3855 s)

to c) The total speed is calculated by

Plug in t = 6.3855. I've got