Over a time interval of 2.16 years, the velocity of a planet orbiting a distant star reverses direction, changeing from +20.9 km/s to 18.5 km/s. Find (a) the total change in the planet's velocity (in m/s) and (b) its average acceleration(in m/s^2) during this interval. Include the correct algebraic sign with your answers to convey the directions of the velocity and the acceleration.