The rate law for the reaction A-->B is rate = k[A] with a rate constant of 2.34 s^-1. If the initial concentration of A is 0.629 mol/L, what is the half-life of the reaction? Answer is 0.296

A particular radioactive undergoes a first order decay with a half-life of 42.6 years. How long would it take for the concentration of this isotope to drop to 30.0% of its original value? Answer is 74.0 years 

How would I do these problems?