Find the defining equation of the function which describes the amount of uranium isotope reamining after x cen

A certain uranium isotope loses 1/8 of its m every century. (Or, phrased another way, the isotope only has 7/8 of its original m remaining after a century. ) We start with 500 grams of this isotope. Find the defining equation of the function which describes the amount of uranium isotope reamining after x centuries.