To shampoo a small dog at Pretty Pooches costs $20. When the owner of Pretty Pooches increases the price to have a small dog shampooed, the number of small dogs shampooed per day decreases. The expression ax+b represents the number of small dogs shampooed in 1 day whenever the price is x dollars per dog. The number of small dogs shampooed per day was 12 when the price given originally was in effect. The number of small dogs shampooed per day decreases by 2 for every $5 increase in price. What are the values of a and b?

How can I show that a = -2/5 and b = 20?