Is Addison has 15 fewer pieces of candy than Ronny does a expression or equation? and why

Let Ronny's candy be x and Addison's candy be y. Then it would read y=x-15.

This is an equation because this is saying that Ronny's amount of candy minus 15 is equal to amount of candy that Addison has. Once there is a useful equal then it is equation.

Let the Addison's candy count be a and Ronny's be r.

Then a = r - 15. This is an equation.

If you had a second equation, you could replace a with the expression (r - 15), which in itself is not an equation.