a) The acceleration that the stone has is the gravitational acceleration and it is always the same (Actually not but assumed to be so)
The gravitational acceleration is 9.81 m/s²
Since this acceleration is always constant, we can use the formula of V = g × t, where V is velocity, g is the gravitational acceleration and t is time.
Let's use this formula: V = g × t = 9.81 × 8.0 = 78.48 m/s
b) Displacement in free falls is calculated by the formula of d = 1/2 × g × t², which is actually derivated from average velocity.
If we use the formula, d = 1/2 × 9.81 × 8² = is found to be 313.92 m
If there is anything you do not understand or anything is wrong, please let me know and I am sorry for my bad English ^^