、。
If you're busy, I'll help.
Literal
Busy-if, I [subject-が] will-help.
忙しければ is the ~ば conditional of the i-adjective 忙しい. The が on 私 is the exhaustive/contrastive が — 'I (specifically) will help,' emphasizing the speaker's offer. ます form signals polite speech. A short, natural offer.