What is the growth rate?

When a batericide is added to a nutrien broth in which bacteria are growing, the bacterium population continues to grow for a while, but then stops growing and begins to decline. The size of the population at the time t (hours) is b=10^7+10^3t-10^2t^2. Find the growth rates at t=0 hours, t=5 hours, and t=10 hours.