"3 times the sum of a number and 5" written as an algebraic expression would be 3(x+5). The process of writing this as an algebraic equation has two parts: forming the base equation and determining what operation to perform on the base equation.
The base equation in this problem is "the sum of a number and 5." This means that a number, x, and 5 must be added, which is written as x+5. The next step is to determine what must be done to x+5. Since the problem says "3 times the sum of a number and 5," x+5 needs to be multiplied by 3. This is written as 3(x+5).