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poor (at); weak (in); not very good (at)
not one's cup of tea; not one's favorite
数学が苦手で困っているので塾に通った。
I'm troubled because I'm poor at math.
私は辛い食べ物が苦手なので、メニューに気を使う必要がある。
Since spicy food isn't my cup of tea, I have to be careful with the menu.
彼女は人前で話すのが苦手だ。
She is not good at speaking in public.