The transitive property states if a=b and b=c then a=c. If two things are equal and one of those two things is equal to another thing, then they all must be equal.
This can be easily solved, especially with a calculator. With the calculator, you can find that is about 6.4807. Multiply that by 2, you get about 13 inches.
If you can't use a calculator, then you can just estimate. Since we know that is 6 and is 7, is in between them, so you can just say somewhere around 6.5.