To consider your question about the positive root, the intermediate value theorem says that if f(x) is continuous on [a, b] and f(a) < 0 and f(b) > 0, then there exists some c in [a, b] such that f(c) = 0. So let a = .30 and b = .31 and show that one of f(a), f(b) is positive and the other negative. Thus a root lies somewhere in that interval.

For the negative root you have no bounds set, so you'll have to do some work to find and a and b with the required properties.

-Dan