use the formula:
average atomic mass= atomic mass1 × abundance1/100 + atomic mass2 × abundance2/100
62.9296 × 69.15/100 + 64.9278 × 30.85/100=
62.9296 × 0.6915 + 64.9278 × 0.3085=
43.5158 + 20.0302=
63.546 amu
so the answer is 63.546 amu
(the element is europium)