Simplifying Positive and Negative Voltage Divider BJT Input
Write the expression for the current through R1 + R2: (V+ - V-)/(R1+R2)

Now find the voltage drop across R2: R2*(V+ - V-)/(R1+R2)

Now find the open circuit Vth at the base connection point:

From ground: V- + (V+ - V-)*R2/(R1+R2)

(The drop across V- plus the drop across R2 is the voltage at the base connection point with respect to ground.)