A scientist is measuring the amount of radioactive material in an unknown substance. When he begins measuring, there are 6.2 grams of radioactive substance. Twelve days later, there are 6.16 grams. After 19 days, there are 6.13 grams. After 46 days, there are 5.92 grams. Assuming that the decay is exponential, find the equation that determines the number of grams remaining after x days.