| $X$ | - 1 | 0 | 1 | 2 | Total |
| $\mathrm { P } ( X = x )$ | $\frac { 3 - a } { 8 }$ | $\frac { 1 } { 8 }$ | $\frac { 3 + a } { 8 }$ | $\frac { 1 } { 8 }$ | 1 |
The probability distribution table of the random variable $X$ is as follows.
\begin{center}
\begin{tabular}{ | c | c | c | c | c | c | }
\hline
$X$ & - 1 & 0 & 1 & 2 & Total \\
\hline
$\mathrm { P } ( X = x )$ & $\frac { 3 - a } { 8 }$ & $\frac { 1 } { 8 }$ & $\frac { 3 + a } { 8 }$ & $\frac { 1 } { 8 }$ & 1 \\
\hline
\end{tabular}
\end{center}
When $\mathrm { P } ( 0 \leqq X \leqq 2 ) = \frac { 7 } { 8 }$, what is the value of the expected value $\mathrm { E } ( X )$ of the random variable $X$? [3 points]\\
(1) $\frac { 1 } { 4 }$\\
(2) $\frac { 3 } { 8 }$\\
(3) $\frac { 1 } { 2 }$\\
(4) $\frac { 5 } { 8 }$\\
(5) $\frac { 3 } { 4 }$