First, simplify where you can: (dropping the units for convenience)

(5426.6 + 15*S)*9.806/(50.752*S) = 0.92

Multiply both sides by 50.752*S/9.806:

5426.6 + 15*S = 0.92*50.752*S/9.806

5426.6 + 15*S = 4.76155823

15*S = -5421.83844177

S = -361.455896118

The unit for S is "kg" since we have to have the same unit for any two terms we add. This also checks when I put "kg" in the denominator for S. Why S is a negative number though, I can't say. Obviously a negative mass makes no sense. What kind of problem is this from?

-Dan