The formula for working out the total interior is:
180(n-2) = 1260
So let’s rearrange by dividing both sides by 180 and find what n is:
n-2 = 7
Then let’s add 2 to both sides to find what n is:
n=9
Each interior angle is 1260/9 = 140°.
The sum of the interior angle and its corresponding exterior angle is 180°.
Therefore, the polygon has 9 sides. :)