Answer
$\color{blue}{a=\frac{S(1-r)}{1-r^n}}$
Work Step by Step
Use the rule $\frac{a}{b}=\frac{c}{d}\longrightarrow ad=bc$ to obtain:
$$S(1-r)=a-ar^n$$
Factor out $a$ to obtain:
$$S(1-r)=a(1-r^n)$$
Divide $1-r^n$ to both sides:
$$\frac{S(1-r)}{1-r^n}=\frac{a(1-r^n)}{1-r^n}
\\\color{blue}{\frac{S(1-r)}{1-r^n}=a}$$
