If you only want to total column B then you should be able to use SUMIF, i.e.

=SUMIF(A:A,"Rent",B:B)

or do you want to add the credits and subtract the debits, in which case either use 2 SUMIFs like

=SUMIF(A:A,"Rent",C:C)-SUMIF(A:A,"Rent",B:B)

or SUMPRODUCT like

=SUMPRODUCT((A2:A100="Rent")+0,C2:C100-B2:B100)