Answer the following questions for the function $$f(x) = x \sqrt{x^2 + 4}$$ defined on the interval $[ -5, 4 ]$.

a.) $f(x)$ is concave down on the open interval .

b.) $f(x)$ is concave up on the open interval .

c.) The minimum for this function occurs at .

d.) The maximum for this function occurs at .

Note: Your answer to parts a and b must be given in interval notation .