A force diagram of the given situation is shown below:
where,
N1 and N2: normal forces on truck and trailer
fg1 and fg2: weights of truck and trailer
d1 and d2: drag forces on truck and trailer
T: tension in the cable
Based on the previous diagram and the given information, you can conclude:
Forces acting on the truck = 5
Forces acting on the trailer = 4
The forces of the system are balanced becasue the speed of the motion for both truck and trailer is constant.
You can notice that drag force on the trailer is opposite to tension in the cable, then:
T = d2 = 600N
T = 600N
The tension in the cable is 600N
Consider that all horizontal forces on the truck are balanced, then, you can write:
th = T + d1
th = 600N + 750N
th = 1350N
The truck generates 1350N of force