How does mass defect relate to nuclear binding energy

Nuclear binding energy is the energy required to split am atoms nucleus into protons and neutrons.Mass effect is the difference between the predicted mass and the actual mass of an atoms nucleus. the binding energy of a system can appear as extra mass, which accounts for the difference.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.