The verb “to answer” is used here in the sense of responding to another’s plea for assistance, like how we might say, God answered our prayer.
1 In figurative language, God responds positively to the heavens, hearing their cry for renewal. Set in motion by God, the heavens respond to the earth by sending down rain. This, in turn, waters the crops which produces an abundant harvest. It vindicates God, who was always Israel’s true Provider and the only Lord over heaven and earth, not Baal (Hosea 2:5).
21 “And in that day I will answer, declares the LORD, I will answer the heavens, and they shall answer the earth,