。
She writes with her left hand.
Literal
She [topic-は] left-hand [with-で] writes.
The で here marks the means or instrument of writing — 'with her left hand' rather than 'at the left hand.' The same で picks up tools (ペンで書く 'write with a pen'), languages (英語で話す 'speak in English'), and other instrumental relations. Left-handedness in Japan was traditionally discouraged — children were often retrained to be right-handed (左利き 'left-handed' was historically corrected to right) — and that history lingers, though attitudes have softened over recent generations. The pared-down structure makes this a clean grammar-textbook example.