hi there,

part i)

well basically this is Newton's second law of motion, F=ma;

where,

F = resultant force acting on the system(car and trailer)

m = mass (car and trailer)

a = acceleration (car and trailer)

therefore applying Newtons second law of motion:

Driving force - Frictional force = ma

5400 - Frictional force = ma

Frictional force = 5400 - ma = R

part ii)

the ratio of mass of the car to that of the trailer= 1200:600 = 2:1

therefore,

the frictional force acting on the car =

the frictional force acting on the trailer =

considering the trailer:

now it is given that the trailer is accelerating at , using Newton's second law of motion again:

tension in the towbar - frictional force on the trailer =

where,

=mass of the trailer = 600kg

tension in towbar = +frictional force on the trailer.

I think you can continue from here. good luck in finding the answer ^-^