I don't understand this steps

If a rock is dropped from a height of 159 ft, its poistion t seconds after it is dropped until it hits the ground is given by the function 3(t) = 16752 + 159. Round values below to 3 decimal places. How long does it take the rock to hit the ground? seconds Find the average velocity of the rock from when it is released until when it hits the ground. feet per second What time after the rock is thrown will its instantaneous velocity be equal to its average velcity? (Apply the Mean Value Theorem) seconds after it is thrown