The best way to do this is to draw out a few polygons and divide them into triangles, then add up the angles. If you start with a square, then a pentagon, then a hexagon, then a heptagon, by the time you get to an octagon, you will see a pattern and can make your inference. Remember that to make an inference is to come to a conclusion based on your observation. So grab a piece of paper and a pencil and start drawing!! You will probably find that they actually did you for this in the Geometry textbook:)