Half-life equation problem; please help!

Question

The half-life of 14C is 5730 years. Back in ancient times, Gandalf the wizard slew a troll under a bridge. 1 g of a tissue sample from a contemporary troll (collected from an organic chemist) showed an activity of 292.9 counts/minute. A 1 g sample of tissue collected from the dead troll's tomb yielded a count of 21.49 counts/minute. How long ago did Gandalf live? Give your answer in years.

Answer 1

m(t) = m(0)*0.5^(t/t½)

21.49 /min = 292.9 /min * 0.5^(t/5730 y)

t = 21,600 y.