Scientists first make observations that raise a particular question. In order to explain the observed phenomenon, they develop a number of possible explanations, or hypotheses. This is the inductive part of science, observing and constructing plausible arguments for why an event occurred.
As indicated earlier, the Bible does not fix the age of the earth, contrary to the claims of Answers in Genesis. Historically, their claim comes from the work of James Ussher, Bishop in the Church of Ireland, from 1625 to 1656.
A straightforward reading of the Bible shows that the earth was created in six days about 6,000 years ago. Radiometric dating uses ratios of isotopes in rocks to infer the age of the rock. Scientists use a mix of observational data and assumptions about the past to determine the radiometric age of a rock.