During the alpha decay, the atom lost two protons and 2 neutrons, making dropping by two in atomic number and by 4 in mass number.
After the subsequent double beta emission, it a proton change to neutron, atomic number increase by one while the atomic mass remain the same,
Therefore after two beta emission the atom will regain the two protons it loss in alpha decay while losing two neutrons.
SOOOOO! the atom loss four neutrons, two in alpha decay and two in double beta emission , the resulting atom (after alpha and two beta emission) will have a mass lost of 4 amu.
which is polonium- 218
