5 The figure at right is the graph of f x the second derivative of a function f x The domain of the function f x is all real numbers and the graph above shows f x for 3 6 x 3 6 30 20 10 A Find all values of x in the interval 3 6 3 6 where f x has a horizontal tangent B Find all values of x in the interval 3 6 3 6 where f x is concave upward Explain your answer