Hmm, Well you've got to work out the angles of the triangle. They should all add up to 360 Degrees if I remember correctly. Your need a pro-tractor of course :p I can't see the picture so dunno what your doing.
Woah, I just looked at the picture, I done that shit in Primary school! Just get a pro-tractor, And measure it and add the other 2 up and take it away from 360. Problem solved