Ejemplo (x-2)/(x-3), x no puede ser igual a 7.
Use command If. Of course the only point in which the function is not defined will not be shown in Graphics View. But you can add it manually, using a suitable symbol for the "hole".
If you enter your function f(x) as
If(x !=7, (x - 2) / (x - 3))
then try to evaluate f(7), GeoGebra yields undefined, as expected.
(!= means "not equal", but you also can select the ≠ symbol in the Input bar dropdown list for symbols)
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