This questions expects us to SOLVE for S, i.e. express the the value of S in terms of other variables, and simplify the answer, if necessary.
We need s to be the subject (on the left hand side), so we need to isolate s.
C=(1/3)m(s+b)
First step, cross multiply to rid of numerical fractions.
C/1=(m(s+b))/3 -> 3C=m(s+b)
If there is a multiplier that does not contain s, divide both side by the multiplier.
3C/m = (m/m)(s+b) -> 3C/m = s+b
We're nearly done. We need to isolate s, so subtract "extra" terms from the side containing s, i.e. subtract b from both sides:
3C/m - b = s + b -b -> 3C/m-b = s
We almost there, but customary practice is to put s on the left-hand side of the equal sign, so just swap sides:
3C/m-b = s -> s=3C/m - b
and that's the answer:
s=3C/m - b
