Suppose that $x$ is a positive multiple of $3$. If $x$ cubed is less than $1000$, what is the greatest possible value of $x$?
The positive multiples of 3 are 3, 6, 9, 12, ...
Which of these numbers, when cubed has the largest value that is less than 1000?
sqrt(1000) = 31.6227 does that give you a clue?