A particle moves according to the law of motion s = t3 - 6t2 + 4t, t >0, where t is measured in seconds and s in feet. a.) Find the velocity at time t. Answer: b.) What is the velocity after 3 seconds? Answer: c.) When is the particle at rest? Enter your answer as a comma separated list. Enter None if the particle is never at rest. Answer: d.) When is the particle moving in the positive direction? When 0