$W$ is not a vector subspace of $R^3$.
Work Step by Step
Let $u=(3,4,5),v=(6,8,10)\in W$. Now, $$u+v=(3,4,5)+(6,8,10)=(9,12,15)$$ which is not an element of $W$, in other words, $W$ is not closed under addition. Hence, $W$ is not a vector subspace of $R^3$.