Create a calculated field on a report

Does anyone know how to create a calculated field on a report? Let’s say there are two fields – “Amount $A” and “Amount $B”, I wanna add a field to calculate the differences between Amount $A and Amount $B → Amount $C = Amount $A - Amount $B

Amount $C = Amount $A - Amount $B should work, (You need " " of course), but what is the issue?

I mean how to create a calculated field(Amount C) on the report? By using which funtion/tool?? to

Just put the control on the form where you need it, then set the source expression to : “Amount $A” - “Amount $B”