Answer
$ d=\frac{l-a}{n-1}$
Work Step by Step
To solve the given equation for $d$, we follow the following steps:
$$
l = a + (n -1)d \Longrightarrow l-a=(n-1)d\\
\Longrightarrow \frac{(n-1)d}{n-1}=\frac{l-a}{n-1}\\
\Longrightarrow d=\frac{l-a}{n-1}
.$$
