I combined the three steps together and the following are my thoughts:
We can set the fixed fee for renting as (f) and an dynamic fee related to days as (d) per day, then we have:
[tex] \left \{ {{f+5d=36} \atop {f+8d=48}} \right. [/tex]
Then we could get f=16, d=4, so we could have the equation as follows:
y = 16 + 4x