Lasers can cool a group of atoms by slowing them down, because the slower the atoms, the

Question:

Lasers can cool a group of atoms by slowing them down, because the slower the atoms, the lower their temperature (see Equation 17-12). A rubidium atom of mass 1.42 × 10−25 kg and speed 212 m/s undergoes laser cooling when it absorbs a photon of wavelength 781 nm that is traveling in a direction opposite to the motion of the atom.
(a) This occurs a total of 7500 times in rapid succession. What is the atom's new speed after the 7500 absorption events?
(b) How many such absorption events are required to bring the rubidium atom to rest from its initial speed of 212 m/s?