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

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