These kind of problems used to really bug me. Keep a cool head and use what you know even though it's letters.
a(c+b)=d
we want to get c alone and everyone else on the other side.
first distribute the a
ac + ab =d subtract the ab term from both sides
ac=d-ab then divide off the a on both sides
c=d-ab/a
so it looks like the last answer is the one you want.
