Calculating atomic mass of an element?

Bromine has two naturally occurring isotopes. Find the percent abundances of 79Br and 81Br given the atomic mass of Bromine = 79.904 amu, the isotopic mass of 79Br = 78.9183 amu, and the isotopic mass of 81Br = 80.9172. (Hint: The sum of the fractional abundances is 1. If x = abundance of 79Br, then 1 - x = abundance of 81Br.)