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She was careless enough to leave her bag on the bus.
Literal
She [topic-は] bus [on-に] bag [object-を] forget [extent of-ほど] careless was.
~ほど here marks the degree of carelessness: 'careless to the extent of leaving her bag on the bus.' The ほど-clause acts adverbially to specify the threshold of the predicate adjective 不注意 ('careless'). 不注意 is built with the negating prefix 不~ on 注意 ('care, attention') — one of the productive 不~ patterns: 不安 ('uneasiness'), 不便 ('inconvenience'), 不思議 ('mysterious').