you need to find the measure of one interior angle. You must find the value of the exterior angle and subtract that from 180 degrees (they are supplementary:_/_).The formula to find the value of an exterior angle is 360/n where n is the amount of sides
360/9=40
subtract from 180
180-40=140
multiply 9 (there is 9 angles)
140*9=1260 degrees
