Geometer's Arthropod's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack without resolving. In that case, use the value of X as that spell last existed on the stack to determine the value of X for Geometer's Arthropod's ability.

In some rare cases, the value of X for the spell on the stack may be different than the value that was chosen when you cast it. For example, if you control Unbound Flourishing (an enchantment with "Whenever you cast a permanent spell with a mana cost that contains {X}, double the value of X.") the value of X may have been doubled. In those cases, use the value of X as the spell exists on the stack at the time Geometer's Arthropod's ability resolves to determine the value of X for Geometer's Arthropod's ability.