Puzzle 45 - Polyominous 9

I have two things to say about this puzzle:

1. For it's size, it was really, REALLY hard to make.

2. As you can see, it has a six and a seven in it (and an upside-down six and seven for symmetry).  What are some of the things you can do with a six and a seven, and what does it make?  The answer is in this puzzle.