A construction worker drops a bolt while working on high-rise building 320 m above the ground. After t seconds, the bolt has fallen a distance of s metres, where s(t)= 5t^2, 0 < (or equal to) t < (or equal to) 8. The function that gives the height of the bolt above ground at time R(t)= 320-5t^2. Use this function to determine the velocity of the bolt at t= 2.