Solve that proportion for ' X ' in the usual way. If you can't remember how to solve a proportion, then THAT's what you have to go look up. It's much more important to you right now than how much energy the sun is putting out.
7.4 x 10⁴ mm is the same size as about 243 feet ! If the tracks on a DVD were that far apart, there would not be room for very many tracks on a DVD. That's not reasonable.
If the tracks are 7.4 x 10⁻⁴ mm apart, then there are about 34,000 tracks in a stripe that's 1-inch wide. I have no idea how they do this, but it's certainly more reasonable than the other choice.
