Angles in Construct work as bearings, with 0 degrees pointing along the X axis and incrementing clockwise. Going by the diagram above, you can find the bearing from B->A using angle(B.X, B.Y, A.X, A.Y) and B-C with angle(B.X, B.Y, A.X, A.Y). However, to get the angle between the two bearings is a bit trickier - you can't just subtract them (sometimes you'll get something like 0 and 270 - the difference is clearly 90 degrees, not 270 as you'd get by subtraction). This requires some funky math which would be a fair bit easier to add as built in expressions to Construct. I'll see if I can add something like that for the next build.
As deadeye said, what's the end result you want? There might be an easier way if its for a game or something.
Cheez: lay off the insults. People are trying to help you.