IMO, bluemagic's graph idea is a very good one. You get to show a 'linear equation' describes a line and that the solution to a set of them is going to at their common intersection, i.e. where the x-values AND the y-values coincide.

It will also build on somethng that your DD has done before - providing a 'seed crystal' for further understanding to bind onto.

Become what you are