Math B problem (please help)

this is a Math B problem for June 2004 # 31. I have the answersheet but i cant figure out how they got the answer.

31 An archaeologist can determine the approximate age of certain ancient

specimens by measuring the amount of carbon-14, a radioactive substance,

contained in the specimen. The formula used to determine the

age of a specimen is A = A02^(-t/5760) , where A is the amount of carbon-14

that a specimen contains, A0 is the original amount of carbon-14, t is

time, in years, and 5760 is the half-life of carbon-14.

A specimen that originally contained 120 milligrams of carbon-14 now

contains 100 milligrams of this substance. What is the age of the

specimen, to the nearest hundred years?