Causes for lawn mower not starting?

The lawn mower: - has gas leaking - new fuel was just put into it - prior to putting new fuel into it, it sat for three years (three winters) with its tank completely empty - does not start when string is pulled - has only been used once I am unsure if the gas leaks regardless of whether or not I'm hoping to... show more The lawn mower:
- has gas leaking
- new fuel was just put into it
- prior to putting new fuel into it, it sat for three years (three winters) with its tank completely empty
- does not start when string is pulled
- has only been used once

I am unsure if the gas leaks regardless of whether or not I'm hoping to narrow down some common suspects based on what has been described. Thanks.
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