this is because if you find the correct area of the purple rectangular shape first, you would have the equation 15 x 9. this equation will give us an answer of 135 mm. our next step would be to find the area of the triangle.
our equation for this will be 15 x 6. this will give us our answers of 90. but because it is a triangle, we will have to divide by two. given is 45. our new equation will now be 135 + 45 given us our sum of 180 cm for the assumed area.
