Answer
The new atomic number is 85 and the mass number is still 218.
Work Step by Step
The atomic number increases by 1 for beta emission, because beta particles carry a charge of -1e. The polonium atom becomes an astatine atom.
The mass number stays the same for beta emission, because beta particles have negligible mass.
This is discussed on pages 628-629. The periodic table on page 216 is also helpful.