Simple circuit analysis question?How to get 1V?
The bottom of the divider is at -5 V because the person who designed the question made it so.  Pretty much by definition, a voltage divider divides the overall voltage present at the ends, whatever they are.


With +/-5 V sources, the overall voltage is 10 V.
Use Ohm's Law to determine the current through the string.
Use Ohm's Law to determine v3.
Because the two power sources are equal but opposite, the circuit Ground, 0 V, is half-way between the two supplies.
Add v3 to the negative power potential to get the output voltage with respect to GND.