Is 得了 the opposite of 不了？ This is from the story 草船借箭 in 三国演义。

诸葛亮：”三天，要十万支箭，你得帮帮我。“
鲁肃：”你不要为难我，我怎么帮得了你？“

In English we can say, "How can I help you?" which may have 2 meanings, depending on how we say it.

Meaning 1. In what way can I be of service to you?

If we put strong emphasis on 'I' and 'you',
Meaning 2. 'I really don't think I am able to help you." 

鲁肃的意思是什么？
Meaning 2 is right.
It is a rhetorical question to express negation.