Please show step by step instructions with equations thank you

3. A stone is thrown vertically upward with a speed of 15.0 m/s from the edge of a cliff of height 90 m. Ignore air resistance. [4+4] a. "Using the downward direction as positive y-axis and using kinematics equations" determine the time to reach the maximum height by the stone. b. "Using the downward direction as positive y axis and using kinematics equations" determine the velocity of the stone with which it strikes the base of the cliff