Find the domain: f(x) = log5(x - 7).
For your question, what Skeeter is saying is that the domain of your function is all the values of x for which we are not taking a logarithm of a negative number.
If we have then for the function to be defined. so that is your domain. Can you take it from here an solve the inequality or do you need further clarification?
The domain is the values of x for which f(x) is defined. When asked to find the domain of a function, I generally look for where the function is undefined, rather than finding where it is defined. For something like your question it's easy to see where it is undefined because logarithms not being able to be taken of negative numbers gives us an inequality and we can infer our domain from that.