how can to get more command on writing formulas in line items
I am not sure the purpose of the community is just asking for the answer to a formula that can be easily googled.
However, what have you tried so far and what is the reason you think you are not getting a result?
Anapedia contains a list of the functions you can use in Anaplan with examples
There's also the planual which provides best practices on modelling. You can also build your experience by completing the Academy training.0