Since y is the original cost, let z represent the final cost. So, we set up the expression, (0.12y + y) - 0.03y = z. That shows first the sale, then the tax. From there, we simplify to get 1.12y - 0.03y = z, then simplify it more to get 1.09y = z. We take away the z, and get that the cost, which is 1.09, or 109%, or y (1.09y).