Simple algebra question - pulling my hair out

Hi,

I have a formula i have to use, depending on the data give in the exam i will have to re-arrange it before using it. Problem is i just cant get it work. I my notes i didnt use a lot of detail and i cant remember how i did it in the class, i thought it was so simple at the time, but now i just cant do it! My lecture says its too easy to need worked solutions, so all i have is the answers, not worked solutions.

f = c ( s - a ) / s ( c - a )

If the question ask for "f", obviously i can do it, but sometimes it asked for "s".

When i try to rearrange, i end up getting "s/s" at some point and the formula explodes in my face.

eg:

c= 0.98

a= 0.89

f= 0.45

s= ? (answer sheet says 0.928)

if you are interested the forumla is for calculating the fraction crystallinity in a polymer sample, from densities, its simple linear relationship.