Discrete signals notation underlined value
An arrow or an underline is used to indicate the value at $n=0$. Example,

$$y(n) = \begin{array}{ccccc}[&amp;9 &amp;8 &amp;\underline7 &amp;6 &amp;5 &amp;]\end{array}$$ means that,
$$\begin{array}{ccccc}&amp;y(-2) = 9, &amp;y(-1) =8, &amp;y(0)=7, &amp;y(1) = 6 &amp;\mathrm{and} &amp;y(2) =5\end{array}$$

In your case, it says that $$\begin{array}{ccccc}&amp;X(0) = 1, &amp;X(1) =-1, &amp;X(2)=2, &amp;\ldots\end{array}$$

Note: No underline or arrow is equivalent to underlining the first value.