please explain

A sample of Carbon - 13 has a half- life of 5700 years. This means after 5700 years, half of it will become Carbon-12. The equation to model this is : t A= Atz 5700 Where A is the final amount, A, is the starting amount & & is the amount of time in years that have passed . If there is 1 kg of C-13 that is 12,000 years old, how much C- 13 was in the sample at the Start 12, 000 years ago