The next step after rotating the first rectangle (in Quadrant III) 90 degrees clockwise about the origin would be to compare the dimensions of the two rectangles. Specifically, you would verify that the lengths of the sides and the overall shape of the rotated rectangle in Quadrant I match those of the second rectangle in Quadrant I. This involves checking whether both rectangles have the same width and height, confirming that they are indeed congruent.
In summary, after rotating the first rectangle, the next step is to compare the dimensions and positions of the rectangles to establish their congruence.