The original ratio between the red and green crayons would be 18:24, but you would need to simplify that to reach your definite simplified answer. Doing this, you find the least common factor between 18 and 24 which is 3 for 18 and 4 for 24.
Answer: 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