Formating an expression for SSRS

How can i limit the numer of decimal places in an expresion for SSRS
=FormatNumber(Fields!SellingQuantity.Value,First(Fields!DecimalsQuantity.Value, “RptParameter”))
i want the output to be in form of #,##0.##

cannot get it right !

Select the Textbox of Expression and update the properties.

No that does not change this field in text box propities looks like it has to be done in the expression

This is the text I used on one of my SSRS forms:
=Format(RunningValue(Fields!DocExtCost.Value, Sum, “Group1”),"#,##0.00")
another one:
=format(cdbl(Fields!DocTotalMisc.Value),"#,##0.00")

thanks worked a treat