Answer 2: Yes, radiometric dating is a very accurate way to date the Earth.
We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes.
For example, the element Uranium exists as one of several isotopes, some of which are unstable.
When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb).
Hope that helps, and please ask if you'd like more details! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer.