A new element, Thorsonium has two isotopes. 67.52\% of the Thorsonium isotopes have a mass of 256 amu. The rest of the sample is Thorsonium - 257 Find the average atomic mass of Thorsonium

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