Answer
$W$ is a vector subspace of $R^3$.
Work Step by Step
Let $W=\{(s, s-t, t) : s \text { and } t \text { are real numbers }\}$, $u=(x_1,x_1-x_2,x_2),v=(y_1,y_1-y_2,y_2)\in W, c\in R$. Now, $W$ contains the zero vector and
\begin{align*}
u+v&=(x_1,x_1-x_2,x_2)+(y_1,y_1-y_2,y_2)\\
&=(x_1+y_1,x_1+y_1-x_2-y_2,x_2+y_2)\in W.
\end{align*}
Also,
\begin{align*}
u&=c(x_1,x_1-x_2,x_2)\\
&=(cx_1,cx_1-x_2,cx_2)\in W.
\end{align*}
Hence, $W$ is a vector subspace of $R^3$.
