Remember your prefixes! They help a lot, because there is a formula for this particular problem. Nonagon refers to a shape with 9 sides.
The number of triangles a polygon has is represented by the formula:
(n-2)
Where "n" is the number of sides. If you ever forget what you subtract "n" by think of a triangle. It has 3 sides, so it is only logical that you should only have 1 triangle (subtract by 2). So, what is the sum of the interior angles of a triangle, 180°. So the formula looks like this:
Sum of Interior Angles = 180°(n-2)
180°(9-2)
180°(7) = 1260° is the sum of interior angles for a nonagon!