Solve:

Ok, so I use the law of cosines, correct?

$\displaystyle cos A = b^2+c^2-a^2/2bc$

Now, when I input this into my ti83+:

$\displaystyle 2017^2+2938^2-2506^2/2(2017x2938)$

I get-1.86075644e13

So, next I am supposed to multiply by the inverse cosine, right? But -1.8 is obviously not right, so what am I doing wrong?