The Calculated formula Mark type allows you to perform calculations of the columns you want.
As explained in the tutorial Create a column, Mark types allow you to define the configuration of the columns and in the case of the Calculated formulas, to perform calculations with the Formula generator and use exclusive features such as apply rounding and Conditional values.
To get started, create a column and assign this Mark type, by scrolling if necessary to find it.
Once configured Formula generator will appear, which lets you easily define the formula in two different ways: using the fx functions that comes by default or set it manually to perform more complex operations.
To manually enter columns into the formula press the drop-down button that appears at the bottom left part and of the list, select the tab you want to include columns. Once defined, click OK to apply it.
Learn how to calculate averages in this tutorial.