The product of a number to the third power and 6 decreased by y

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