~のが得意 (good at doing)

The standard pattern for expressing skill: verb-の + が得意 = 'is good at doing X / one's specialty / good at X.' The の nominalizes the verb. 得意 ('skilled, forte') is a na-adjective with a nuance of 'one's strong suit' rather than just general skill. Contrasts with 苦手 ('poor at, not good with'). Parallel to ~のがうまい.