Finding accerlation in this problem

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To give a 16 kg child a ride, two teenagers pull on a 3.4 kg sled with ropes, as indicated in Figure 5-26. Both teenagers pull with a force of F = 53 N at an angle of 35° relative to the forward direction, which is the direction of motion. In addition, the snow exerts a retarding force on the sled that points opposite to the direction of motion, and has a magnitude of 57 N. Find the acceleration of the sled and child.

picture: http://i232.photobucket.com/albums/ee319/reddevilzofah/05-23alt.gif

Okay, I've tried to solve this problem and I added all the Fx. So I did 16+3.4a=-53cos35+57 Solved for a, but the answer was wrong. Can someone tell me what I did wrong and how to fix it?

Thanks!
 

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Could you please show us each step of your work, so we can look for where you might have made a mistake?
 
16+3.4a=-53cos35+57 and solved for a. Thats that work. Is the equatio wrong?
 
keweezz said:
16+3.4a=-53cos35+57 and solved for a. Thats that work. Is the equatio wrong?
Two mistakes:
(1) The total mass is 16 + 3.4. So the left side should have parentheses: (16 +3.4)a.
(2) There are two teenagers pulling, not just one.