little more about boron told,
say in a network once you have found all values of current, then there will be some questions to find Vab, etc, ie: voltage of (a ) relative to (b), in such cases the way to find out Vab is start from the reference point (b) and go via any path on the network towords (a) add all voltages which is on the same direction and subtract voltages that are oposite. so when you pass some resistors depends on the direction of current flow the voltage is added or substracted.
normally these voltages are maked with arrows indicating high side by head and low side by tail near every component to eleminate any confusion during this. so once the arrow is in the same direction its given +ve sign (add) and if its oposite to the path then its given -ve sign (substract)