Voltage divider with a negative rail
This is your equation for Vout in terms of V+,V- R1 and R2:

VO = V+ - ((V+ - V-)/(R1+R2))*R1

In short, just find the total voltage, divide by the total resistance to get current, then find the voltage drop across one of the resistors (I used R1) and use that to get VO.  If you found the voltage across R1 then do V+ - VR1 or if you found it across R2 then do V- + VR2. 

Oh, and don't use a divider like that to power any circuitry, if you actually need the rail buffer with  an opamp. Also, don't use that circuit as a voltage reference,  with 200 Ohm  total R you will be throwing away lots of current.



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