Final answer:
The expression for the greater number is a + 8, which is found by adding the given difference of 8 to the lesser number a.
Step-by-step explanation:
The student has given us an equation to work with: The difference between two numbers is 8, and the lesser number is known as a. To find the expression for the greater number, we simply need to add the difference to the lesser number. Therefore, the greater number can be expressed as a + 8.
Let's illustrate this with an example. If the lesser number, a, is 5, then to find the greater number, we add the difference of 8 to 5, giving us 13 as the greater number.