1. **State the problem:** We need to find an expression for the total cost when ordering $y$ books, where the first book costs 15 and each additional book costs 12. 2. **Understand the pricing:** - The first book costs 15. - Each additional book (after the first) costs 12. 3. **Formulate the expression:** - If $y=1$, cost is 15. - If $y>1$, cost is 15 for the first book plus 12 for each of the remaining $y-1$ books. 4. **Write the expression:** $$\text{Cost} = 15 + 12(y - 1)$$ 5. **Simplify the expression:** $$\text{Cost} = 15 + 12y - 12$$ $$\text{Cost} = 12y + 3$$ 6. **Interpretation:** This means the total cost for $y$ books is $12y + 3$ dollars. **Final answer:** $$\boxed{12y + 3}$$