Please illustrate how to rotate triangle A(2,0), B(7,0),C(7,2) 90 degrees.
About what point? Clockwise or counterclockwise?
Plot the first points on a graph and you will see many possibilities. The fact that this is a right triangle aligned with axis makes it simple. One possibility would be (7,0), (9,0) and (7,5) (For clockwise rotation about point B).