chemistry help needed?

Question

A biochemist studying the breakdown in soil of the insecticide DDT finds that it decays by a first-order reaction with a half-life of 12 yr. How long does it take DDT to decay from 305 ppbm to 17 ppbm in a soil sample ("ppbm" is parts per billion by mass)?

Answer 1

(17/305) = (2 to the x/12 power)

Solve for x.