How to combine variables with strings in a formula with vba

Question

I'm struggling with writing the correct grammar of a formula using the property "xlA1", I want to use the next formula: IF(X311=0," ",X311), I want to change "311" of the X for a variable, but I don't know how to write it, I tried this:

inputFormula = "=IF("X" & a "& "=0," "," & "X" & a &")"

but I get an error.

Thanks for your help!.