If dy/dt=-3y and if y=1, what is the value of t for which y=1/3? I got y=e^(-3t) but I wasn't for sure if it was correct.
Let's see..
With your y(t)= e^(-3t),
dy/dt = (-3)e^(-3t) = -3y
You do not state your initial or boundary value conditions in a meaningful way. If y = 1, it cannot also be 1/3, no matter what x is.
Make sure you have stated the problem correctly