Answer
(a) Nowhere ($\varnothing$)
(b) $x \in (-\infty,0)$
(c) Nowhere ($\varnothing$)
(d) $x \in (0,\infty)$
Work Step by Step
The function is always decreasing, so (a) and (c) both have answers of "for no x" or $\varnothing$. For negative x, the rate of change is becoming more negative, making it concave down. For positive x, the rate of change is becoming less negative, making it concave up.