Experiment a little.
If one point on P is (0,1), then
That point on Q is (2,1), and
That point on R is (2,-1)
Are you sure it has to be a 180º rotation? How about a simple reflection through a point? Does it make any difference?
Of course, it's always fortuitous to find a point that doesn't go anywhere.
If one point on P is (x,y), then
That point on Q is (1-(x-1),y), and
That point on R is (1-(x-1),-y)
Necessarily, then, we have y = -y and x = 1-(x-1). What do you get for x and y? What does that tell us?