So to put this in terms, we are going to have the cost of the equipment is equal to, a fixed rate + (a daily rate * the amount of days the equipment is rented). Without the fixed rate and daily rate, it is difficult to create an equation, but it will look something like this:
y=w+mx
y=how much it costs to rent the gym equipment
w=the fixed rate
m=the daily rate
x=the amount of days the equipment is rented.