The adjective good is always followed by the preposition at when talking about skill or ability.
Structure:
Subject + be verb + good + at + noun / verb + ing
Examples:
1.She is good at English. 2.Ali is good at mathematics. 3.He is good at swimming. 4.They are good at solving problems. 5.She is good at playing the piano.
Good at = Skill or ability
Remember:
Good + at + Verb-ing / Noun
Examples: