Step-by-step explanation:
[tex]s = \frac{a}{a - r} [/tex]
multiply both sides by a - r
[tex](a - r)s = a[/tex]
divide both sides by s
[tex]a - r = \frac{a}{s} [/tex]
subtract a from both sides
[tex] - r = \frac{a}{s} - a[/tex]
invert the signs
[tex]r = - \frac{a}{s} + a[/tex]
