How many sides does a polygon have if the sum of its interior angles is 1,260º?

One angle = 360 / n 
Let x be the angle and n be the number of sides. 

x = 360 / n 

nx = 360 

now, let's use this: 

(2n - 4 )*90 = 1260 
2n - 4 = 1260 / 90 
2n - 4 = 42 / 3 
2n - 4 = 14 
2n = 18 
n = 9 

x = 360 / 9 
x = 40 degrees for each exterior angle 
so the interior angle's measurement is 140 degrees 

So 9 would be the answer

This came from a previous post seen here: 

Hope this helps :)