Answer
$$
\frac{2S-na}{n}, \ \ 4\frac{1}{4}
.$$
Work Step by Step
To solve the given equation for $l$, we multiply both sides by $2$ and then we follow the following steps:
$$ S=\frac{n}{2}(l+a) \Longrightarrow 2 S=n(l+a) \Longrightarrow 2 S=nl+na \\ \Longrightarrow 2S-na=nl\Longrightarrow l=\frac{2S-na}{n}.$$
Now, we have
$$
l=\frac{2S-na}{n}=\frac{2\times 96.875- 15\times8.666}{15}=4.250=4\frac{1}{4}
.$$
