。
She was living a peaceful life in the countryside.
Literal
She [topic-は] countryside [in-で] peaceful [possessive-な] living [object-を] was-doing.
平穏な暮らし ('peaceful living') uses the na-adjective 平穏 (tranquil/peaceful) — a more literary word than 静かな (quiet) or 穏やかな (calm). 暮らしをしていた is the standard pattern for describing a lifestyle one is living. The ~ていた progressive past places this as an ongoing state at some point in the past.