Math Is Fun Forum

In my physics class, my teacher introduced me to dimensional arrays, but explained them very poorly; naturally, I was hoping you could help me.

This is the problem I need help with.  It's purpose was to convert a light year into inches, and LY stands for light year.  Could someone tell me how the problem is solved to get 3x10^17 inches, and the process they took?

Thanks in advance,
Wladislaw