Try doing it "cleanly" :

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

Multiply by s(c - a):

c(s - a) =fs(c - a)

Do the multiplications:

cs - ca = fcs - fas

Get the terms with s together:

fas - fcs + cs = ca

Factor out the s:

s(fa - fc + c) = ca

Finish off:

s = ca / (fa - fc + c)

That's it !