The original price is $9000 per unit. The unit is x, so if you buy x units, you pay 9000x.
The original price function is
R(x) = 9000x
The discount is 7 cents per unit bought, so if you buy x units, the discount is 9x in cents, or 0.09x in dollars. This discount is subtracted from the original price, so the discounted price is