[Arsim]

Effective nuclear charge of carbon 2p is 3.14. How do you calculate it?

[AdiDex]

there is no such formula which can give you exact value of effective nuclear charge (as far as I know) .

You can use Slater's Rule to find out .
But you should realize that that these rules are simplified generalizations based upon the average behavior of the various electrons .Although the electronic energies estimated by Slater's rules are often not very accurate, they permit simple estimates to be made and will be found useful in understanding related topics such as atomic size and electronegativity.