basically get a shape
eg we will work with triangles,, add angles to the inside,, ensuring they add up to 180 (because angles in a triangle add up to 180)
now that we have the angles,, extend each side with a line to show the exterior angles.
we know that angles on a line add up to 180,, so to find the external angle,, we can use our internal angle
you would use:
[ 180- internal angle]
do this for all external angles
add all your answers together,, should equal to 180
