Rootha's ability will check as your beginning of combat step starts to see if you've cast an instant or sorcery spell this turn. If you haven't, the ability won't trigger at all. It's not possible to cast an instant or sorcery spell during your beginning of combat step in time to have the ability trigger.

For spells you cast with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.