The accompanying table shows the number of milligrams, y, of a dose of a cold medication that remains in the body x hours after the dose has been ingested.

x(hrs) 2, 3, 5, 7, 9

y(mg) 11, 6.7, 2.9, 1.5, 0.9

Determine an exponential function y=ax^b that best fits the given data. Then estimate constants a and b to the nearest tenth. Find to the nearest minute when the amount of the drug that remains in the body first falls below 0.5 milligram.