4Fe+3o2+xH2O becomes 2Fe2o3xH2o

I want to know how does the 'x' come in between ....

When the Fe comes contact with oxygen and moisture(h20) , hydrated ferric oxide Fe2O3.nH2O also commonly called rust is formed.

I think you have confusion in 'n '. Well, here n stands for any positive integer. Rust can have any number of water molecules depending upon the extent of the process.

Eg. Fe₂O₃. 3H₂O or Fe₂O₃. 7H₂O . In fact no. of water molecules is not fixed so we generally use 'n '. Hope you understand what i want you to understand.