Question 52894
You've done everything just fine up to {{{Sb=rL-a+Sa}}}, so let's start from there.  Now subtract rL from both sides and we get {{{Sb-rL=Sa-a}}}.  Now let's factor the right side to get {{{Sb-rL=a(S-1)}}}.  Now we can solve for {{{a}}} bu just dividing both sides by (S-1):  {{{(Sb-rL)/(S-1)=a}}}.