I'm making a first person shooter and I'm trying to use trigonometry to create arms for the enemy to be able to hold a gun. Here's an images to help show what I'm trying to do.

I guess another way i could explain this is that I have an acute triangle (I think that's what it's called. In other words, a triangle that has three sides of different length) and I know that length of all three sides. How to I find the angles of that triangle?