Answer
$$
\frac{2S-nl}{n}, \ \ 31
.$$
Work Step by Step
To solve the given equation for $a$, we multiply both sides by $2$ and then we follow the following steps:
$$ S=\frac{n}{2}(l+a) \Longrightarrow 2 S=n(l+a) \Longrightarrow 2 S=nl+na \\ \Longrightarrow 2S-nl=na\Longrightarrow a=\frac{2S-nl}{n}.$$
Now, we have
$$
a=\frac{2S-nl}{n}=\frac{2\times 575- 25\times15 }{25}=31
.$$
