In this sentence ajudar is used with the auxiliary verb poder. So, just like in English, it stands in the infinitive. (e.g. “He can help”, as opposed to " He helps" without the auxiliary verb “can”). In the sentence “Posso ajudá-la?” it might not be obvious, but ajudar is in the infinitive as well. If a verb ends with -r, -s, or -z, the last consonant is dropped and an “l” is added to the pronouns a, o, as, os.
I hope I got everything right. If not, hopefully one of the moderaters can help. 