Which formula we will use to solve this problem?

[keskiverto]

When you see a keyword (or possible keyword), you can try to search for it in documentation (or web). In this case the word is "static_cast", which is a rather distinct word and thus more likely to get decent hits in a search.

For example: http://en.cppreference.com/w/cpp/language/static_cast

It is one of C++ style casts. Explicit type conversions. C has only one casting syntax:
intValue = int(dblVal);
... and that is not identical in functionality.

Further notes on them (more to point than the wordy reference doc):
http://stackoverflow.com/questions/1609163/what-is-the-difference-between-static-cast-and-c-style-casting