You can't "find" the atomic mass of iodine-129 because it's a defined value.

Here's why:

* Isotopes: Iodine-129 is a specific isotope of iodine. Isotopes of an element have the same number of protons (defining the element) but different numbers of neutrons. This difference in neutrons affects their atomic mass.

* Atomic Mass Unit (amu): Atomic mass is measured in atomic mass units (amu). 1 amu is approximately equal to the mass of a proton or neutron.

* Defined Value: The atomic mass of iodine-129 is 128.905 amu. This value is determined by international scientific bodies and is used for consistency in calculations.

Therefore, you don't "find" the atomic mass of iodine-129; you simply look it up in a reliable source like a periodic table or a reference book.