They found the bone in the rock. Scientists look at the rock and can tell the rocks age. They assume that the bone is about the same age as the rock it was found inside.

And how did they determine the age of the rock?

There are many methods of determining rock age. A common one is finding signature fossils that are known to only exist between certain ages. Putting the rock in a machine to record its magnetic field and correlating this with pole reversals on earth. Using isotope ratios (not just radiocarbon dating), chemical composition, identifying how the sediment was laid down by rivers or the sea etc.

This may be as close to ELI5 as it can be. https://simple.wikipedia.org/wiki/Radiometric_dating

Scientists look at the bone and can tell the bones age. ((Just kidding))