The goal is to find the right triangle, depicted in red, in a four-sided polygon with respect to the diagonal. The four-sided polygon's vertices are known.
Initial attempts: The angle formed by the diagonal and the positive x-axis is calculated. I used x1 of v1 > x1 of v3 to identify the red triangle if the angle was found within the first quadrant.
Draw a line parallel to the given diagonal from both of the other two points, and determine which one has an x-intercept greater than the other.