It might be a typo you put, but you divide by 10^10, and not 10^-10. Your final units of Sbound should be 10^-6
Sbound/Sfree should be from 0 to 1.5 and Sbound should be in units of 10^-6 Mol/L If you forget to put all Sbound in 10^-6, you're slope will come out as -0.333333 instead of 333333...
I'm probably in your class UCD MCB123
We can help each other out if you like, but I sort of don't do my homework until the last minute, as you can see.
Unit Conversion
You want Mol/L for concentration of S,
but you must start with DPS.
So DPS/(Activity of DPS), where Activity = 10^9 DPS/uMol...