Eeq=(e1r2-e2r1)/r1+r2. Which formula is this? Plz explain

Dear student,
It is formula for equivalent emf of the two cells.

We have two cells E1 and E2 with internal resistance r1 and r2 connected in parallel with a resistor R. 
Let, I1 and I2 be the current through E1 and E2 respectively. 
By applying KCL(Kirchhoff's Current Law), 
I = I1 + I2 
Let there resultant EMF is Eeff and resultant resistance be Req. 
By KVL(Kirchhoff's Voltage Law) 
Potential across R = Eeff - I*Req 
Potential across upper branch = E1 - I*r1 
Potential across Lower Branch = E2 - I*r2 
After solving we will get 
Eeff = (E1/r1 + E2/r2)*Req 
and 
Req = r1*r2/(r1 +r2)

Eequa=(E1R2+E2R2)/(R1+R2)

In the above question as the cells have opposite polarities, so a minus sign comes in between.
so Eeq=(e1r2-e2r1)/r1+r2

Regards