I have a problem here which is:

Find the sum of allnexterior angles of an-sided convex polygon, where n is any positive integer > 3. Show all the working details and reasons.

I need a solution for the general case. So far I know that it is 360 degrees but I am completely unsure of how to start my proof. I am currently 13 years old, so I do not know very advanced maths. Could you please help me here? Thank you very much!