In custom reports how do I add two columns together to total in a different column?

Make column "c" a formula field and just enter the formula +. That should sum it up for you.

For on-the-fly custom columns in a report, you can actually build a <Custom Column> in the report which can make calculations.  This field only exists in the report you build it in; but is hugely useful when you want to build a formula that you can then use as a filter in the report for temporary use. 

To use this feature, click on the checkbox labelled [Define a calculated column].
You choose a formula-type and then write your formula in the box.  You also give your calculated column a name, which will appear in your columns list under that name, so you can place it wherever you want in the report easily.

For filtering, the option is a <Calculated Column> found at the bottom of the field list, which will allow you to set filters on the output values of your custom-calculated column.

This is a great temporary solutions without building new fields that are not required for permanent use.  You can only create 1 calculated column in a report.