There are several useful documents available at
http://frc-manual.usfirst.org/. The "Arena Layout and Markings" is one of them, and has the answer to your question.
The back of the pyramid is 9 feet from the center line, which is 27 feet from each wall. The pyramid is 8 feet wide, so the closer edge of the pyramid is (27 - 9 - 8 =)
10 feet from the Alliance wall.
The field is 27 feet wide, and the pyramids are centered between the sides. So, the pyramid is ((27 - 8) / 2 =)
9.5 feet from the sides.