How do you calculate negative and postive voltages in a grounded circuit?
Ground is just our reference point, against which we measure voltages.  Current is not flowing into or out of it (in this example).
Because the resistance above and below the ground point is equal, we know that current passing over these resistances will drop the same amount of voltage.
The total resistance of the circuit is represented between points A and D...this is where 6 V will be dropped.
Point C is midway through this resistance.  Therefore, there is 3 V drop between A and C and another 3 V between C and D.
Using Point C as our reference for &quot;zero volts&quot;, point D will be -3 V and point A  will be +3 V.  This, of course, shows our total of 6 V across the whole circuit.