Let's see if we can figure this out. Start by listing the information in a way that let's you use math on it. We'll do that by using variables for each of the 3 sides
x=shortest
y=middle
z=longest
we know the total length is 25in so:
x + y + z = 25in
We also know...

Think of a negative exponent as really putting the number raised to the exponent on the wrong side of a fraction. I mean by that if I have 3^-2 that is really 1/3^2. or if I have 1/(3^-2) that is really 3^2. to get rid of negative exponents just flip the number being raised...

the domain is what the input numbers can be. normally this is just the x, but if the function "does something" to the x then the domain becomes what the "something" did to the x.
In your problem: the x can be anything from 4 to 18;
so the domain of f(2x) is anything...

You are trying to show how much your phone bill will be (COST) when you use it (m minutes). Draw your axis for your graph. Place C on the vertical axis (up and down) since that is what you want to know. Place m on the horizontal axis (left-right) since this is what you are going to do use the phone...