Hard chemistry question ?

Naturally occurring iodine has an atomic m of 126.9045. A 12.3849-g sample of iodine is accidentally contaminated with 1.00070 g of 129 Iodine, a synthetic radioisotope of iodine used in the treatment of certain diseases of the thyroid gland. The m of 129 I(iodine) is 128.9050 amu