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.