Answer
$$
\frac{2V-lwD}{lw}, \ \ 3.00
.$$
Work Step by Step
To solve the given equation for $d$, we multiply both sides by $2$ and then we follow the following steps:
$$ V=\frac{1}{2}lw(D+d) \Longrightarrow 2 V=lw(D+d) \Longrightarrow 2 V=lwD+lwd \\ \Longrightarrow 2V-lwD=lwd \Longrightarrow d=\frac{2V-lwD}{lw}.$$
Now, we have
$$
d=\frac{2V-lwD}{lw}=\frac{2\times226.8-9\times 6.3\times 5}{9\times 6.3}=3.00
.$$
