Over a time interval of 2.16 years, the velocity of a planet orbiting a distant star reverses direction, chaning from +20.9 km/s to -18.5 km/s.
a) Find the total change in the planet's velocity in m/s
b) Find 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 aceleration.