Answer is: the atomic mass of the second isotope is 64.95 amu (Cu-65).
Ar(Cu) = 63.546 amu; the average atomic weight of copper.
Ar(Cu-63) =62.92; he average atomic weight of copper-63.
ω(Cu-63) = 69.15% ÷ 100% = 0.6915.
ω(Cu-second isotope) = 100% - 69.15%.
ω(Cu-second isotope) = 30.85 % ÷ 100% = 0.3085.
Ar(Cu) = Ar(Cu-63) · ω(Cu-63) + Ar(Cu-second isotope) · ω(Cu-second isotope).
63.546 amu = 62.92 amu · 0.6915 + Ar(Cu-second isotope) · 0.3085.
Ar(Cu-second isotope) · 0.3085 = 63.546 amu - (62.92 amu · 0.6915).
Ar(Cu-second isotope) · 0.3085 = 20.037.
Ar(Cu-second isotope) = 20.037 ÷ 0.3085.
Ar(Cu-second isotope) = 64.95.