We would compare the coordinates of corresponding vertices on the red and black triangles. The vertex at the top of the black triangle and the vertex at the bottom of the red triangle correspond to each other. We would compare them.
For black triangle, coordinate is (1, - 3)
For red triangle, coordinate is (- 1, 3)
If a vertex with coordinate, (x, y) is rotated 180 degrees counterclockwise about the origin, the new coordinate is (- x, - y). By rotating (1, - 3) 180 degrees counterclockwise, it becomes (- 1, - - 3) = (- 1, 3). This corresponds to the coordinate of the corresponding vertex of the red triangle. Thus, the rotation that maps the black triangle onto the red triangle is 180 degrees counterclockwise rotation about the origin