for this: {0,3,6,9,12}

S={x|x $\displaystyle \in$ N , x is multiply of 3, x $\displaystyle \leq$ 12}

S={x|x is multiply of 3 and 0 $\displaystyle \leq$ x $\displaystyle \leq$ 12}

Which one is correct and more valid?

and how to write for this one: {m,n,o,p} ?