Can someone explain me how to get the answer

Question

You are driving your car, and the traffic light ahead turns red. You apply the brakes for 3.30 s, and the velocity of the car decreases to +4.85 m/s. The car's deceleration has a magnitude of 2.82 m/s² during this time. What is the car's displacement?

Answer 1

If the car was decelerating than it would have an acceleration of -2.82 m/s².

To calculate the initial velocity we can use the kinematics equation "v_f = v_i + at" so:

4.85 = v_i + (-2.82)(3.30)

4.85 = v_i + -9.306

v_i = 9.306 + 4.85 = 14.156 m/s

To find the displacement you can use the kinematics equation "x = v_it + 1/2at² " so:

x = (14.156)(3.30) + 1/2(-2.82)(3.30)²

x = 46.7148 - 15.3549

x = 31.3599 meters but rounding 2 3 significant figures, x = 31.4 m