4mg+2H2O
Please learn the differences between a shool, school, and a school subject.
mg means nothing. I assume you mean Mg.
Mg + 2H2O ==> H2 + Mg(OH)2
The expression "4mg+2H2O" doesn't seem to represent a valid chemical equation. It appears to be a combination of a numeric value (4mg) and a chemical compound (2H2O).
If you are looking for a balanced chemical equation or reaction involving water (H2O) and another substance, please provide more specific information or context so that I can assist you further.