how do you work out a missing angle using trigonometry

Geno is right

If you want to find an angle you have to use inverse trig 

inverse trig is the same as arc trig.   

So on the web2 calc  inverse sin is asin.      

Is your missing angle in a triangle?

If so, is it a right triangle, or just any kind of triangle?

And, what parts of the triangle do you know?

If you post back with more information, we can help.