Nucleus a having Z = 17 and equal number of protons and neutrons...

In $A, Z = 17$ and have equal numbers of protons and neutrons i.e. nucleons$= 2 \times 17 = 34$

So, for $A$,

Total binding energy $B{E}_{A} = 1 . 2 \times 34 = 40 . 8MeV$

Another nucleus $B$ mass number $= 26$

So, total binding energy of $B$ is $B{E}_{B} = 1 . 8 \times 26MeV$$= 46 . 8MeV$

Difference of $BE = B{E}_{B} - B{E}_{A} = 6MeV$