Not a puzzle as such.
The sum of the internal angles of a triangle is 180°
For a square it's 360°
For a pentagon (5 sides), the sum is 540°
There's a formula for working it out for shapes with more sides. Where n is the number of sides, the sum of the internal angles is (n-2) x 180º
So a straight line is just 1 side obviously which means that the internal angles of a straight line total (1-2) x 180º which is minus 180°
No prizes if you can see why that's a flawed calculation