Question

How to find the exact value of 6pi without a calculator

Answer 1

you can't
6pi is irrational
it would only give you up to a certain amount of digits

Answer 2

what i'm saying 6pi is the exact value
the calculator will give you an approximation

Answer 3

But, you can get arbitrary close to the exact value by hand, using this formula:
\[\frac{\pi}{4} = 1-\frac{1}{3}+\frac{1}{5}-\ldots\]