You should find that the derivative is entirely negative whenever x < 0. This suggests that the function f(x) is decreasing on this interval. So that takes care of part (b).
The interval x < 0 is the same as -infinity < x < 0 which then translates to the interval notation (-infinity, 0)
Similarly, you should find that the derivative is positive when x > 0. So the function is increasing on the interval (0, infinity)
