Three Boxes

Three boxes sit next to each other on a flat table. Their masses are:
m_{1}=45.2 kg., m_{2}=22.8 kg. and m_{3}=34.3kg. figure

a) What horizontal force F is needed to push the boxes to the right,
as one unit, with acceleration of 1.32 m/s^{2}?

b) Find the force exerted by m_{2} on m_{3}.

c) Find the force exerted by m_{1} on m_{2}.