First, you minus both sides by 16x. This will get you -3y = 3 - 16x
Then, you divide both sides by -3. Hence on the right hand side you will get 3 - 16x over -3, which can be split into (-16x/-3) + (3/-3). By simplifying this you can get 16/3 x - 1.
20 represents the cost of the first sheet and then we add plus 4n which represents each additional sheets which leads out to A, our average cost per sheet.