The function y=f(x) has second derivative . Find the intervals on which y=f(x) is concave down. Give your answer using interval notation. Use infinity U for union if required.

I know this is a pretty simple question, but I couldn't make it to this lecture. I'm pretty sure your supposed to find the points of inflection and then put them in a table to find where it's concave down but every time I try this I get an incorrect answer. Thanks Alot!