The total interior angles of a hexagon = 720°
Each time we add a side (triangle to square, square to pentagon, pentagon to hexagon) we add another 180°.
Triangle to hexagon
Triangle 

Square 

Pentagon 

Hexagon 


`180°` 
`+` 
`180°` 
`+` 
`180°` 
`+` 
`180°` 
`=` 
`720°` 
The total internal angles of a hexagon = 720°
Now divide the total internal angle by the number of corners.
i.e. divide 720° by 6.
`720^circ/6=120^circ`
1 

2 

3 

4 

5 

6 


`120°` 
`+` 
`120°` 
`+` 
`120°` 
`+` 
`120°` 
`+` 
`120°` 
`+` 
`120°` 
`=` 
`720°` 