For which interval of x is the function decreasing and nonlinear?

Between x = -2 and 0.
Explanation: Let's first look for an interval where the gradient is decreasing. That is, if we were given the function for each curvature or line, the derivative would be negative.

We see that only two places have a decreasing gradient (between 0 and -2 and 2 and 4).

Now, the question asks for a non linear line. So, we want to look for a curve instead of a line and thus, the only viable interval is between -2 and 0.