Variable names grayed out in Trend Line dialog

The Trend Line dialog has options to change the names of the X and Y variables. However, this option is grayed out. Why is that? How can I change the names?

image description

The variable names relate to the [ ] Show Equation option (i.e., they define what is written instead of x and f(x) in the trend line’s f(x) = 1.234 x² - 5.678 x + 9.012); and that latter option is not available when you have Moving Average selected.

I confused matters by showing Moving Average in the example. However, the variable name options are grayed out no matter which regression type is selected. You alluded to the solution I was looking for: “Show Equation” needs to be checked before the variable name options are activated.
Thanks again for your help.