The mass of 126c is exactly 12 u. what is the total binding energy of this nucleus?

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