How to calculate the angle using work, displacement and 2 forces?

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

Given:
Work = 12000 J
Weight: 800 N
displacement: 200 m
Force: 120 N
angle = ?

2. Relevant equations
W = Fd

W = Fd cos θ

cos -1 (W/Fd) = θ

3. The attempt at a solution

First I added the 2 forces together.

800 + 120 = 920 N

calculated work; 200 m * 920 N = 184000 J

then used the second formula;

cos -1 (12000 J /184000 J) = 86.3°

but the answer in my book says the answer is supposed to be 60°….

help me please.

http://ift.tt/1eL2rju

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