A shape with 7 sides is a heptagon
A shape with three sides is a triangle.
you call a two dimensional shape with six sides a hexagon
Triangle is a shape with three sides.
A shape with 8 sides is an octagon.
A regular shape.
a shape with nine sides is called nonagon
It is an irregular shape
A shape with straight sides is called a polygon. But it can have 3 or 4 sides as well.
octadecagon or octakaidecagon
If you say this shape is 18 sided i would guess it has 18 sides. ;)
A shape with 5 sides is a pentagon