Sommetime we require to use values from textboxes in SSRS for some calculations or data hiding and some other manipulations. Textbox or any other object on report design is an item of the report and can be accessed through ReportItems!TextBox.value
This can be used as on textbox expression =ReportItems!Amount.Value - ReportItems!Amount1.Value
This can be used as on textbox expression =ReportItems!Amount.Value - ReportItems!Amount1.Value