You can
put this solution on YOUR website!This one seems confusing at first, but you just need to get the two r terms together so you can factor them out, here is how I did the problem

Mulitply (r-1) to get it out of the denominator

Distribute the S through r-1

now we need to get both r terms on one side of the equation so we can take the GCF of 'r' out
so we subtract rL from both sides
add S to both sides
Factor out the r on the left
Divide both sides by S - L
Hope that helps, Mr. C