It depends on what the shape is
I’m not sure what happened while you were writing this, but if I’m reading it right, “My” has 18 books. If “My” has 3 times as many as 6, that also means 3 times 6, which is 18.
Basically you just need to multiply the two fractions together to get the depth of the turtle. Therefore the turtle is 3/12 miles deep or 1/4 mile deep.