As the current encounters the 5 Ω resistance, voltage is dropped across the resistor’s ends.
The polarity of this voltage drop is negative (-) at point 4 with respect to positive (+) at point
3. We can mark the polarity of the resistor’s voltage drop with these negative and positive
symbols, in accordance with the direction of current (whichever end of the resistor the current
is entering is negative with respect to the end of the resistor it is exiting: