You can figure out the voltage across a resistor in a series of resistors by considering that they all have the same current flowing through, and their voltages add up. The result is the following equation:
V_out = V_in*R_out/R_total
Here, V_in = 10 V, R_out = 3 kOhms, and R_total = 9 kOhms, so V_out = 3.333 V.
