Math Word Problem: Exponential Decay.?

Carbon-14 is a radioactive isotope used to date objects. The element's decay can be modeled by the function A(t)=A(0.999879)^t, where A(t) is the amount of carbon-14 currently present in an artifact, A is the initial amount of carbon-14, and t is the time in years. An oil painting supposedly painted by Pico is presented for examination and found to contain 92.4 percent of its original carbon. Write an equation that can be used to determine the age of the painting.