12 here represents the mass number, which is the sum of neutrons and protons. Since carbon has 6 protons, then it means that there are: 12 - 6 = 6 neutrons. The formula for binding energy is:
E = Δmc²
Let's compute the apparent mass of 126C.
mass of each proton = 1.00727647 amu
mass of each electron = 0.00054 amu
mass of neutron = 1.0086654 amu
m = 6(1.00727647) + 6(1.0086654) + 6(0.00054) = 12.099
So,
Δm = 12.099 - 12 = 0.099
Therefore,
E = (0.099 amu)(3×10⁸ m/s) = 29.7 MJ