After experimenting, I now understand the suggestions from @Daniel_Grubbs and @pirsquared, and see how either could be used to solve the problem posted. The suggestion from @pirsquared is the best fit, however, so that is the one I’ve adopted. Both suggestions have moved on my knowledge of Desmos, so again thanks!
Apart from tidying, that probably completes the activity unless someone has a suggestion for improving the ‘heavy handed’ table construction on the second screen, as posted in a separate thread.