resultant force on the piston at the end of stroke.

1. The problem statement, all variables and given/known data

my question is on part a , the ans is 4260N . but my answer is double of the ans given. why i’m wrong? the working is shown in the photo.

2. Relevant equations

3. The attempt at a solution

Attached Images
File Type: jpg IMG_20140616_083027[1].jpg (145.6 KB)

http://ift.tt/1qkSZs8

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