Two tickets to an ice skating performance costs $36. For five tickets it costs $90, and for nine tickets it costs $162.

what is the constant of proportionality and what does it mean?

So for any triangle, you can divide it into four congruent triangles by connecting the midpoints of each side. You would divide the rectangle into half, to form two congruent triangles. When you multiply the area of 1 of the triangles, it gives you the area of both triangles, and so the area of the triangle as a whole.