Don't be embarrassed about asking.... it's all good. I didn't know what it meant till after I had the GN for a while, and I had a turbo car before this one (Mr2).
Anyway, A = area, and R = radius... you know how the turbo looks like a snail, or like a tube wrapped in a circle? Well if you were to take a "slice" of the turbo so that you had a ring... and you determine the area of that ring (area of a circle), you will have your A. Then measure the radius from the middle of the wheel to the middle of this sliced section, and you will have your R. Divide A by R, and you will get your answer. I bet that was extremely confusing! It would be best to look at a picture of what the heck I'm talking about, I'm sure it's on-line somewhere.
Good luck.