The function is f(x) = [2/3] [6^x]
It is an exponential function whose range is all the positive values. This is, for any value of x that exponential function yields a value that is greater than 0. The graph never crosses the x-axis.
When you reflect the function over the x-axis, you flip the graph over x, and you can see that the range of the new fucntion iis all the negative values, this is all the real numbers less than zero.
Answer: all the real numbers less than zero.