PHYSICS!!! PLEASE HELP WITH CIRCUITS?

In neuroscience, it is sometimes useful to use a constant current source (CCS), which instead provides a fixed amount of current through itself. In this case, we have to use our physical principles in order to calculate the voltage drop across the source. Suppose we have a fixed current source that always provides a current of I0 = 10-6 amps. For the three circuits shown below, find the voltage drop across the current source. Each resistor has a resistance R = 2000 ohms. (If you prefer, you may leave your answer in terms of the symbols I0 and R.)