ANSWER
B(r,0)
or
B(0,s)
EXPLANATION
In rectangle ABCD, the coordinates of A are (0, 0) and of C are (r, s).
When we name the triangle in a clockwise direction.
Then B must bear the y-coordinate of A and bear the x-coordinate of C.
This gives us (r,0)
When we name the triangle in an anticlockwise direction.
Then B must bear the x-coordinate of A and bear the y-coordinate of C.
This gives us (0,s)
