1. **State the problem:** A florist needs 12 roses for each round table and 36 roses for a centerpiece. We want to write an equation for the total number of roses $y$ depending on the number of round tables $x$. 2. **Identify variables:** Let $x$ = number of round tables. 3. **Write the formula:** Total roses $y$ equals roses for round tables plus roses for centerpiece. 4. **Express mathematically:** $$y = 12x + 36$$ 5. **Explanation:** For each table, 12 roses are needed, so multiply 12 by $x$. Then add 36 roses for the centerpiece. 6. **Final answer:** $$\boxed{y = 12x + 36}$$