If you learn to use the LaTex markup tags and that will help make things more clear in the future. You would enter \frac{a}{1-r} between the "math" tags.
Anyway,
Multiply both sides by 1-r
Use distributive law
Subtract both sides by s
Almost there, now divide both sides by -s (minus s)
All the cool kids move the negative sign up from the denominator
Distributing the minus sign (which is really a -1)
Now rearranging the numerator so the positive term comes first