Should I ask her out and if so how do I do it?

So I've been thinking about asking out a former female coworker (Im a guy). We worked together for several years and a couple years ago both left to take jobs in completely different fields. I've talked to her a couple times briefly since then over fb messenger just seeing how she's doing but it's been close to a year since then and I don't have her phone number. I just don't want to make it awkward but I also know I need to show confidence if I'm going to have any chance with her. What do I do?
Should I ask her out and if so how do I do it?
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