Applying for a job and it requires me to say previous work's salary. Help?

I'm applying to a job and upon clicking through it, not only does it require me to state what salary I want per year, but it also requires me to state the salary I started with and ended with at my previous employers.

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I'm unsure how to exactly answer this.

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For the salary asking what I want per year, I put "negotiable", however, at a loss for what to put for previous salary. Or should I just steer clear altogether?

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