What is asked is the binding energy. The formula is
E = c² Δm
In a lithium-6 atom, there are 3 protons and 3 neutrons.
The mass of the nucleus is
3(1.672622) + 3(1.674927) = 10.035344
The binding energy is
E = (3x108)² (10.035344 - 6.015123)
E = 3.618 x 10^17 J
