Given data
*The value of the acceleration due to gravity is a = 10 m/s^2
*The given time is t = 6 s
*The initial speed of the rock is u = 0 m/s
The relation for the velocity, acceleration, and time is given by the kinematic equation of motion as

Substitute the known values in the above expression as

The speed of the rock in MPH is converted as

Hence, the speed of the rock is v = 134.21 mph