The function f(x)=40x-80 models the number of visitors a website received x days after it was created. which is the most appropriate domain for function?

Hello there!

When finding the domain of a function, the only things you need to watch for are...
1. Variables under even radical signs
2. Variables in the dominator of fractions
3. Logarithms

Since this function is linear, there is nothing that would stop any real number from being part of the domain.

This means that the domain is all real numbers. (-∞,∞)

I hope this helps!
Best wishes:)