The concentration of a drug in the body decreases exponentially after a dosage is given. In one clinical study, adult subjects averaged 15 micrograms/milliliter (mg/mL) of the drug in their blood plasma 1 hr after a 1000-mg dosage and 2 micrograms/milliliter 6 hr after dosage. a) Find the value k, and write an equation for an exponential function that can be used to predict the concentration of the drug, in micrograms/milliliter, hours after a 1000 mg dosage.