EDIT
Answer: x≤6
Explanation: the total number of hours (x) is less than or equal to 6 hours.
Because if you check my math below where I misread the question and gave a function... LOL
You will see that 7 hours would be too many.
So very sorry for the initial confusion!!
Answer: f(x)=12X-15
Explanation:
X represents the number of hours.
F(1)= 12(1)-15 = $-3 or free. In real life I'm sure there'd be a requirement that you rent at least 2 hours to get the discount