Variable to reduce a fraction or
expression to its lowest
terms.
simplify
operation

Lets see what we're working with here

-18(s) + 24 - [-9] (s) = ?

Take out your "like terms" 
  -9(s) + 36

First,  we have to 'solve' the number -9 which is equal to 0

Solve this one too (only if you want to it to make it easier) (s) - 4 = 0
         We have to add a positive four to your sides (+4)

We have found one solution in this equation

The value of "S" is 4