[b]Define this and try to keep it unsimplified.[/b] [i](Check the toolbar for that)[/i][br][br]f(x) := (x^2 - 9)/(x - 3)

[b]Now use the command sequence to perform several substitutions at once. [/b] [url=https://geogebra.github.io/docs/manual/en/commands/Sequence/](Click here to know more about sequence)[br][/url][br][br]Sequence[Substitute[f(x), x = a], a, 1, 5][br][br][br][br]

[b]Try the same with:[/b][br][br][br]Substitute[Simplify[f], x = 3]