First some definitions:
A. Isotopes - atoms with the same number of protons, but a different number of neutrons (same element, different isotopic mass).
Carbon can exist the isotopes carbon-12, carbon-13, and carbon-14. They both have 6 protons (or else they wouldn't be carbon), but a different number of neutrons.
C-12 has 6 protons and 6 neutrons
C-13 has 6 protons and 7 neutrons
C-14 has 6 protons and 8 neutrons
B. Radioactive nucleus - a nucleus that spontaneously changes and emits (releases) energy. This occurs spontaneously: by itself and with no outside energy required. Many isotopes do it naturally.
All nuclei with more than 84 protons (Polonium and up) are radioactive. As well as those with with more neutrons than protons Carbon-14 is radioactive
Balancing: The sum of the isotopic masses (top numbers) are equal on both sides of the equation.
The sum of the atomic numbers (the bottom numbers) also are equal on both sides of the equation.