Place any three numbers on the vertices of a triangle:
5 / \ / \ / \ 8-------4
Calculate their differences, going one way:
5 / \ -3 / \ -1 / \ 8-------4 4
Place another number inside the triangle:
5 / \ -3 / \ -1 / 15 \ 8-------4 4
Add the centre number to a vertex, and multiply the total by the opposite edge. Do this for each vertex. Add them together.
$(15+5)\times4=80$
$(15+8)\times-1=-23$
$(15+4)\times-3=-57$
$80-23-57=0$
You got $0$. Why?