This is rather a sneaky question. You don't know exactly what numbers you can use and what numbers not.
For example, are you allowed to use x = - 3. The answer is yes
f(-3) = 2*7^(-3) = 2/7^3 = 2/147.
How about 0? If there is going to be a problem, zero is usually the number that causes the problem.
f(0) = 2*7^(0) = 2 * 1 = 2 and in this case 0 is just fine.
How about positive numbers? No they are all good.
So the range is -∞ < y < ∞
