Then think about what will give me a negative number for b....none. You might think -1 or -2, but those are merely and and so on and so forth.
Similarly no x value will also give you 0, some might try zero, but
So you can see that the domain has to be restricted for positive values.
An easy way to find what values will work is
and solve. Remember it cannot be 0 so it's a strict inequality.
If you are given numerous logs you can do the same for each and if you end up with a quadratic you can compare the solutions to see which fit the criteria.
Hopefully this helps.