The binding energy of deuteron (21H) is 1.15 MeV per nucleon and an alpha particle (42He) has a binding energy of 7.1 MeV per nucleon. Then in the reaction
21H+21H→42He+Q
the energy Q released is approximately
23.8 MeV
Binding energy of 21H=1.15× number of nucelons = 1.15×2=2.3 MeV.
Total binding energy of reactants = 2.3 + 2.3 = 4.6 MeV.
Binding energy of 42He=7.1× number of nucleons =7.1×4=28.4MeV.
Therefore , Q= 28.4 - 4.6 = 23.8 MeV.