This week in precalc 11 I learned about the sine rule.  

 

Lets say you are given this triangle, and you are looking for the top angle. The sine rule is  

sinA  sinB    sinC 

—- = —- = ——- 

a           b          c 

 

But you only need two of them to make it work, and you need at least 3 pieces of information. 

We can then label the sides and make an equation 

Sin21      SinB 

——- =  —— 

4              5 

 

We multiply the sin21 by 5, then divide by 4 to get 0.4479… 

And the last step is to inverse sin, then multiply by the previous answer. 

If done correctly, the angle will be 27 degrees.