stuck on this on
$\displaystyle s = a / l-r$
solve for r
also how do you make a divide a straight line in this forum?
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,
$\displaystyle s= \frac{a}{1-r} $
Multiply both sides by 1-r
$\displaystyle s(1-r) = a $
Use distributive law
$\displaystyle s - sr = a $
Subtract both sides by s
$\displaystyle -sr = a - s $
Almost there, now divide both sides by -s (minus s)
$\displaystyle r = \frac{a - s}{-s} $
All the cool kids move the negative sign up from the denominator
$\displaystyle r = \frac{-(a - s)}{s} $
Distributing the minus sign (which is really a -1)
$\displaystyle r = \frac{-a + s}{s} $
Now rearranging the numerator so the positive term comes first
$\displaystyle r = \frac{ s - a}{s}$