紅葉 (kōyō: autumn colors; leaves changing color) crimson + leaves
One word appears in both sentences. I’ll define it only once.
庭の木がすっかり紅葉しました。
Niwa no ki ga sukkari kōyō shimashita.
The leaves of the trees in the garden have completely turned red.庭 (niwa: garden)
木 (ki: tree)2、3週間もすれば木の葉は紅葉するでしょう。
2, 3-shūkan mo sureba ki no ha wa kōyō suru deshō.
The leaves will turn red in two or three weeks.3週間 (san-shūkan: three weeks)
week + interval
葉 (ha: leaf)This sentence showcases 葉 in both its kun-yomi form, ha, and its on-yomi form, YŌ.