This is the second problem in which you say you have an exam coming up but have no idea how to do a problem. That's a very bad situation!

Half life MEANS "time until only half is left"- if you start with , after one "half life" you will have left.

Put that into your given equation: when t is the half life- which you are told is 2.045 years: . The " "s cancel leaving . Solve for k by taking the logarithm of both sides.

Once you know k, use instead of and the value of k you just found: and solve for t.