Can someone please help me with the following question, I need to make sure I'm using the right method.

Two trucks, A and B both weight 10,000 Kg and are attached together by a light horizontal coupling. The accelaration of the system is 0.2 ms^-2 and the resistive forces on trucks A and B are 500 N and 300N respectively. Find PN (the driving force).

So I did F= ma +800

F= 20,000*0.2 +800

F= 4800N

Now however I am told that the resistive force on truck A has increased to 2000N but PN has remained the same and I have to find the new acceleration. How would I work this out exactly?