3 Resistor Voltage Divider Formula
Your circuit can be solved using the suoerposition principle. First set Vbatt equal to 0 and calculate the effect of Vm which is simply a voltage divider consisting of R5 and the parallel combination of R6 and Ro.  Then set Vm equal to 0 and calculate the effect of Vbatt which is simply a voltage divider consisting of R6 and the parallel combination of R5 and Ro. What you have is basically an adding circuit where the output is a weighted sum of Vbatt and Vo.