Product meaning multiplication of numbers, and your numbers are: n to the third which is an exponent, therefore n^3. And the other number is 6 so put that together and get n^3 * 6. Now the word decreased by means that the number after by is being subtracted from the prior number, so that means:
n^3 * 6 - y
or you can use parenthesis to make this clearer.
(n^3 * 6)-y