How do I go about finding the exact value
of ?

You have to know the formula 



Substitute  for A and  for B:



Now you have to know that

 and that 

So substitute these:



or



Your teacher may accept it that way, but s/he 
probably would prefer that you rationalize the 
denominator:









Factor 2 out of the numerator



Cancel the  in the bottom into the
 in the top, getting -1 in the top:


 
Remove the parentheses:



or you can save one sign by writing the
second term first



Edwin