cos^-1( 7/14)

You should learn how to use the calculator here:cos^-1( 7/14)=60 degrees.

7/14 = 1/2

According to special angles table, cos $60^{\circ}$ = $\frac{1}{2}$

$\therefore cos^{-1}(\frac{7}{14})=60^{\circ}$

Here is a special angles table: http://www.mathwords.com/t/trig_values_of_special_angles.htm