Child's Toy

(a) Look at all three cars as one system.
\sum F_{x}  =  m_{total}a_{x}
6.5N  =  (3.1kg+2.4kg+1.2kg)a_{x}
0.97\frac{m}{s^{2}}  =  a_{x}.
(b) Now look at only the third car.
\sum F_{x}=m_{3}a_{x}=(1.2kg)(0.97\frac{m}{s^{2}})
and you can solve to get F=1.2N pointing to the right.
 
(c) Look at only the second and third cars. 
F=(m_{2}+m_{3})a_{x}=(2.4kg+1.2kg)(0.97\frac{m}{s^{2}})=3.5N.