whats error

= IIF(PREVIOUS(Fields!firstname.Value +"  " + Fields!middlename.Value
+  " "  + Fields!lastname.Value + " & " +Fields!Address.Value) =
Fields!firstname.Value +"  " + Fields!middlename.Value
+  " "  + Fields!lastname.Value + " & " +Fields!Address.Value

and (previous(Fields!RX_.Value) = Fields!RX_.Value),
"",

Fields!firstname.Value +"  " + Fields!middlename.Value
+  " "  + Fields!lastname.Value + " & " +Fields!Address.Value)

i am getting error "value and is not a  member of ...................."