The cost of renting a car is described by the function
f(x) = 30x + 25 in which f(x) is the total cost and x is the
number of days. In the equation which number represents
the flat fee (constant)?

30

Explanation:

The constant is the slope or m. Since this equation is y=mx+b the 30 is m

25 is the fee you pay no matter what, however the 30 is the fee that reacts to the number of days. No matter if it's 30 days or 2 the amount per day stays the same, this is why it is the constant.

It's called a flat fee because the price does not change for every x no matter the amount of days.