CHEMISTRY COMMUNITY

A substance A decomposes in a first-order reaction and its half life is 355 s. How much time must elapse for the concentration of A to decrease to (a) (1/8)[A]0
I plugged in the values to the integrated rate law equation for first order reactions but I got 30.36 mins and I'm not sure where I went wrong. Which equation should I be using?

I also had a problem with this question but for part c "How much time must elapse for the
concentration of A to decrease to (c) 15% of its initial concentration"

I used the half life of a first order rxn to find k, then solved for t using the first order integrated rate law, however I cannot seem to get rid of the negative in the equation. What am I doing wrong?