Ohm's law is not difficult, but you don't understand the problem, especially when the circuit is complicated. You don't know whose voltage the voltmeter is measuring. Ohm's law is like the law of pressure. It seems so simple that two numbers divided by one equals another number. The difficulty is that you don't know the circuit, not ohm's law. Now you only need to understand the circuit diagram and find out whose voltage and current are measured by the series-parallel circuit and the voltage ammeter, and you have succeeded more than half. The specific operation is to find a workbook with answers and compare it with the exercises, so you really won't ask questions.