"Sum" means addition, so the sum of a number x and 3 is
x + 3
"Product" means multiplication, so the product of 8 and x + 3 is
8(x + 3)
You need the parentheses to show it's the product of 8 and x + 3.
If you wrote only 8x + 3, that would mean the product of 8 and x, then you add 3 to it, which is different from your problem.
