A1000 kg car pushes a 2000 kg truck that has a dead battery. when the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 4500 n.

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F = ma 

In this case m = m car + m truck; a is assumed constant and not need for the answer. 

F = F accelerate the car + F accelerate the truck. 
a is the same for both, so the F accelerating the car is ½ half the force accelerating the truck. 

F truck = 2 x F car 

F = F car + 2 F car = 4500 --> F car = 1500; F truck = 3000 

The car is pushing against the truck with 3000 N force.