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.
