I need to find area of a polygon without using built in functions

I need to create a code that will allow me to find the area of a polygon given any set of coordinates . We are supposed to start with one point and work around the polygon basically forming triangles with the starting point and somehow adding them together to finally form the total area of the polygon. I am not allowed to use any built in functions. My hint was that I would need to use cosine rule in my code. I cannot figure out how to do this and need help badly.

[Roger Stafford]

I showed you how to do this in your earlier request at #108495. It forms triangles with the origin rather than at your starting point, but it wlll still give you the correct area.