law of cosines

112.61986494804282480511 is what I come up with. Not sure how many decimals you need to round to, but that's it.

my final equation was:

$400 = 169 + 121 - 286\cos C\\ -286\cos C = 400 - 169 - 121\\ \cos C = -\dfrac{110}{286}\\ C = 112.6198649^{\circ} = 112^{\circ}37'11.51''$

 

the arccosine of a negative number is in the range 90° < x $\leqslant$ 180°