Question 474554
first let's use law of cosines

{{{cos C = ( 21^2 + 14^2 - 28^2 ) / (2 * 21 * 14)}}}

{{{cos C = ( 441 + 196 - 784 ) / 588}}}

{{{cos C =  -147/588}}}

{{{cos C =  -0.25}}}



The inverse function of cosine.

Basic idea: To find cos-1 (-0.25), we ask "what angle has cosine equal to -0.25?" 

The answer is:

{{{C = 104.5}}} °