What is the digit $a \mathrm{in}$

$$
a 000+a 998+a 999=22997 ?
$$
Performing the addition

$$
\begin{array}{r}
a \\
a \\
a 998 \\
+\quad a 999 \\
\hline \square 997
\end{array}
$$

we find $\square 997=22997$, where $\square=a+a+a+1$.

Therefore, $22=a+a+a+1$. Thus, $a=7$.