for a hexagon total sides =6

sum of all interior angle of a regular polygon = (n-2)180

for 6 sides this is 4*180 = 720 degree

measure of 1 internal angle=720/6 = 120 degree

Any 1 of the 6 vertex can be extended to form a straight line

Straight angle =180 degree. This leaves us with 180-120= 60 degree for a single external angle

60*6=360 degree = sum of exterior angles (B)

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This is my response to the question and may be incorrect. Feel free to rectify any mistakes