Voltage divider output / OP amp
It would be more clear what you're asking if you included a schematic. 

But I think what you are looking for is \$V_o=V_- + (V_+-V_-)\frac{R_2}{R_1+R_2}\$ which is the formula for the output from a voltage divider connected between voltages \$V_-\$ and \$V_+\$.

You can derive this formula yourself using the principle of superposition.