Binding energy is given by E=Δmc2
where Δm is the mass defect.
The energy per nucleon of the nucleus having mass number 242 is 7.6MeV.
The initial binding energy is,
BE=242×7.6MeV
The energy per nucleon of the nucleus with mass number 121 is 8.1MeV.
Therefore, binding energy is
BE′=2(121×8.1MeV)
The gain in the binding energy is
BE′−BE=(8.1–7.6)×242MeV=121MeV
=121MeV