When the unknown letter appears both inside
and outside a trig ratio in an equation, 
then there is no way to solve such an equation 
by any of the  methods of algebra. It can only 
be estimated using a graphical calculator.  
Of course we can see right off by inspection that 

 is one solution.  

However there is another solution, which I
got from a TI-84.  Graph these:




with calculator in RADIAN mode.  Then find their
other point of intersection, which is
approximately



Edwin