When a party has the burden of proving any claim [or affirmative defense] by a preponderance of the evidence, it means you must be persuaded by the evidence that the claim [or affirmative defense] is more probably true than not true.
You should base your decision on all of the evidence, regardless of which party presented it.
Links
[1] https://www.ce9.uscourts.gov/jury-instructions/sites/default/files/WPD/1.06_civil_2017.wpd