excel 2003 visual basic intermediate course - Why does my worksheet show =SUM(R[-2]C:R[-1]C)?

The reason why you have this result =SUM(R[-2]C:R[-1]C) showing in your formula and your column headings are displaying 1 2 3 instead of A B C etc., is because your settings have been changed in Tools, Options.

To reset so that your formula reads as =SUM(A1:A2) and your column headings display A B C;

Select Tools, Options,
General Tab,
under Settings uncheck the box R1C1 Reference style.
Click OK
Your worksheet should be back to normal settings.