The most common ways are to just answer affirmative (sí) and either parroting back the same verb (in your case, sí, podré/puedo) or sometimes using a generic verb like hacer (e.g., sí, lo haré).
The trick is that English allows answering questions with just an auxiliary verb, but Spanish doesn't, and in case, as you've seen, what's done with an auxiliary verb simply isn't separable at all because it's just one word in Spanish. Note what happens when we switch to the periphrastic future:
¿Vas a poder terminar la tarea antes de las 6?
— Sí, voy. (wrong)
— Sí, voy a poder. (okay)
— Sí, podré. (okay)
— Sí, puedo. (okay)
¿Vas al cine esta noche?
— Sí, voy. (okay, because ir isn't being used as an auxiliary)
Similar will happen with other constructions:
Have you eaten today? Yes, I have.
¿Has comido hoy?
— Sí, he. (wrong)
— Sí, he comido (okay)
— Sí, ya comí (okay).