Answer:
Voltage difference is 13 volts
Explanation:
If voltage A (
 ) is -8 volts, and voltage B (
) is -8 volts, and voltage B (
 ) is 5, the voltage difference is calculated via the formula
) is 5, the voltage difference is calculated via the formula 

When estimating a difference between quantity A and another quantity B it is custom to perform the difference built as the last quantity mentioned, minus the first one.