The average atomic weight of this element can be calculated as follows:
average atomic weight =
(atomic weight of first isotope)(its percentage of abundance) +
(atomic weight of second isotope)(its percentage of abundance)
atomic weight of first isotope = 68.9 amu
its percentage of abundance = 0.604
atomic weight of second isotope = 70.9 amu
its percentage of abundance = 0.396
So, just substitute in the above equation to get the atomic weight of the element as follows:
atomic weight = (68.9)(0.604) + (70.9)(0.396) = 69.692 amu