How do you Prove that the sum of the exterior angle so formed is 360 if the sides of a triangle are produced in order?

All exterior angles of any polygon add up to 360 degrees because angles on a straight line add up to 180 degrees as for example an equilateral triangle has 3 equal interior angles of 60 degrees and so 180-60 = exteror angle of 120 degrees.

Therefore 3 times 120 = 360 degrees.