Hi,
I am not sure what you want to achieve, but Plot is a procedure which needs fixed bounds for the range, such as:
p[x_] := x^2 + 2 x - 3;
Plot[p[r], {r, -20 , 20 }]
![enter image description here](/c/portal/getImageAttachment?filename=ScreenShot2015-04-23at13.43.50.png&userId=48754)
If you want to try different ones you might want to use Manipulate:
Manipulate[Plot[p[r], {r, -2 RR , 2 RR }], {{RR, 1}, 0.1, 20}]
You will get a Slider and get get something like this:
![enter image description here](/c/portal/getImageAttachment?filename=manipulate.gif&userId=48754)
Hope this helps,
Marco