Solving for r is the same as isolating it. Since we have (1-r) being multiplied by L, we want to divide it out so we can work with (1-r), so:
[tex] \frac{S}{L}=1-r [/tex]
Now, we want to make -r positive, so we multiply both sides by negative 1:
[tex] -\frac{S}{L}=r-1 [/tex]
Now, to isolate r we want to add 1 to both sides, so:
[tex] 1-\frac{S}{L}=r [/tex]
and we're done.
