Hi, I'm new, I hope this is the right forum for this!

I am trying to re-create a model I wrote years ago, (lost the source code), One requirement is an algorythm that will calculate the angle, between two points on a plane, from the x,y coordinates of the start and finish; I will be writing in Pascal or Object pascal, but I am NOT asking for code, just the algorythm. However, if you do NOT have the design but do have code, I will be happy to look at that