Your post (" f(x) = 2/3(6)x ") would be clearer and less ambiguous if you'd please format it as follows:
f(x) = (2/3)(6)^x. The (2/3) shows that 2/3 is the coefficient of the exponential function 6^x. Please use " ^ " to indicate exponentiation.
Start by graphing f(x) = (2/3)(6)^x. The y-intercept, obtained by setting x=0, is (0, 2/3). Can you show that the value of f(x) is (2/3)*6, or 4, at x=1, (2/3)*6^2, or 24, at x = 2, and so on? What happens if x becomes increasingly smaller? The graph approaches, but does not touch, the x-axis.
If you complete this graphing assignment, then all you'd have to do is to flip the whole graph over vertically, reflecting it in the x-axis. You'll see that the graph never touchs the x-axis. Therefore, the range of this flipped graph is (-infinity, 0).