Is the Force on Box A Correctly Calculated?

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[SOLVED] Newton's Third Law problem

Homework Statement

Three boxes, A, B, and C, are positioned next to each other on a horizontal, frictionless surface. An applied force acting on box A causes all the boxes to accelerate at $$1.5m/s^{2}$$

. Calculate the force that box B exerts on box A.

http://img70.imageshack.us/img70/8836/thirdlaw2cf8.png​

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Homework Equations

$$F_{A on B}$$= $$-F_{B on A}$$
F=ma

The Attempt at a Solution

What seems to be the issue is how my teacher solved this problem. I have only realized this problem, so I can't have asked her personally. What she did is isolate box B and find the $$F_{net}$$ acting on it. This is the calculation she did:

$$F_{net}$$= $$F_{A on B}$$-$$F_{C on B}$$
$$F_{net}$$= $$m_{B}a$$ -(-7.5 N)
$$F_{net}$$= (10 Kg)($$1.5m/s^{2}$$) + 7.5 N
$$F_{net}$$= 23 N

However, what confused me is the two negative signs she put infront of the 7.5 N. I think only one would suffice to take into consideration that $$F_{C on B}$$ is a negative value, being that its direction is left.

I solved this problem taking a different approach. I found $$F_{net}$$ of box B and C, and added them together. Here are my calculations:

$$F_{net}$$= $$m_{B}a$$ + $$m_{C}a$$
$$F_{net}$$= (10 Kg)($$1.5m/s^{2}$$) + (5.0 Kg)($$1.5m/s^{2}$$)
$$F_{net}$$= 23 N

Could someone please confirm whether my method of solving the problem is correct or not?​

 

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Anyone?

Is their something wrong or confusing with my post that nobody has replied yet?

 

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...

...anyone please?