I don't get this..

Find all intervals of increasing&decreasing&all relative extrema. Find all intervals where f is concave up or down. Find all points of inflection.

f(x) = 6/(x^2+3)

I am lost on this, no idea how to solve it, I took the derivative and I get my x as sqrt(6i).

How do I get the concept of these problems? Sometimes I'm able to do them and other times I don't get the concept..