MHB Mechanics- connected particles

[Shah 72]

A car of mass 2000kg pulls a caravan of mass 1200kg along a straight horizontal road. The resistance on the car is 20N and the resistance on the caravan is 80N. The max possible driving force from the car's engine is 1900N. The tow-bar will break if the tension exceeds 680N.
a) Find the max possible driving force before the tow-bar breaks.
b) Find the maximum possible acceleration.

 

[Shah 72]

So I did -20-680-80=3200a using F=m×a
a= -0.244m/s^2
Again using f=m×a
F-680-20=2000×-0.244
F=212N
Max force= 1900-212=1688= 1700N correct to 2sf
Max possible acceleration will be a=1700/3200=0.5m/s^2
Is this the correct way of doing this??