Correct answer: −9 m/s
To find the velocity when acceleration is zero, first calculate acceleration by differentiating the displacement equation twice: a = d²s/dt² = 6t - 12. Set a = 0 to get t = 2 s. Then, find velocity by differentiating displacement once: v = ds/dt = 3t² - 12t + 3. Substituting t = 2, v = 3(2)² - 12(2) + 3 = -9 m/s.