hide a div with javascript in materialize css

I want to know how to do to show a div that is hidden by the class "hide" in materialize. If I run in this way Id_div.className -="hide"; it works perfectly but if I use the variable as below the function stops working showing the undefined hide message.

function mostrar(Id_div){
 var x=Id_div;
 x.className -= "hide";
}