Answer
$\approx 80.4^0$
Work Step by Step
From the law of cosines
$120^{2}=100^{2}+85^{2}-2\times 100\times 85\times \cos \theta \Rightarrow 17000\cos \theta =100^{2}+85^{2}-120^{2}=2825$
$\Rightarrow \cos \theta =\dfrac {2825}{17000}\approx 0.167\Rightarrow \theta \approx \cos ^{-1}\left( 0.167\right) \approx 80.4^0$
