I guess that n=17 because it's a demonstration of the associative property of numbers and multiplication.
A*(B*C)=(A*B)*C. In words, it means no matter what order you decide to do multiplication (or addition) the result is always the same. The operation in parentheses is done first then the result is multiplied by, or is used to multiply, what's outside the parentheses, but the result is the same no matter which you do first. I think that's what the teacher was trying to tell you. The large dot to me means multiplication. Similarly, A+(B+C)=(A+B)+C. This doesn't work for division and subtraction and you can't mix addition and multiplication. For example, A*(B+C) is not the same as (A*B)+C.
By the way, perhaps for extra credit, you should know that A*B=B*A, which is the commutative property of numbers and multiplication. When you combine the two properties you can see that A*(B*C)=(A*C)*B or A+(B+C)=(A+C)+B and other combinations, as long as you don't mix addition and multiplication in the same sum. These rules apply in algebra, too, where variables (unknowns) represent numbers.
I hope this important concept is a bit clearer now, and that you continue to improve your grades.