Answer: average atomic mass of the element is 35.4536 amu and the element is chlorine.
Step-by-step explanation:
Mass of isotope 1 = 34.969
% abundance of isotope 1 = 75.77% =

Mass of isotope 2 = 36.965
% abundance of isotope 2 = (24.23)% =

Formula used for average atomic mass of an element :

![\text{ Average atomic mass of an element}=\sum[34.969* 0.7577)+36.969* 0.2423]](https://img.qammunity.org/2019/formulas/chemistry/high-school/k08z1ojbtr2la1scawt19v3ujh6pwkmg4q.png)

Therefore, the average atomic mass of the element is 35.4536 amu and the unknown element will be Chlorine.