Question 185376
It's possible that you are working in radian mode where you actually want to work in degree mode. 



{{{c^2=13^2+20^2-2(13)(20)cos(27)}}} Start with the given equation.



{{{c^2=169+400-2(13)(20)cos(27)}}} Square each term.



{{{c^2=169+400-2(13)(20)(0.891)}}} Evaluate the cosine of 27 to get 0.891 (if you get a different value, then you're in the wrong mode).



{{{c^2=169+400-463.32}}} Multiply



{{{c^2=105.68}}} Combine like terms.



So you're answer is confirmed to be {{{c^2=105.68}}}