Final answer:
The distributive property is used in this equation to simplify it.
Step-by-step explanation:
The property used in this equation is the distributive property. The distributive property states that when multiplying a number by a sum or difference, you can distribute the multiplication to each term inside the parentheses. In this case, the 3 outside the parentheses is being multiplied by both x and 5. So, the equation becomes 3 * x + 3 * 5 = 3x + 15. This step simplifies the equation and allows for further solving.
Learn more about Distributive property in mathematics