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
