To find the open circuit voltage, work the problem like this:
Open circuit, so the current in the 6 ohm resistor is zero, therefore the voltage drop across this resistor is zero
=> Open circuit voltage = voltage across the 2 ohm resistor.
Current around the loop V/R is 10V/(2 + 3 ohms) = 2A.
Voltage across 2 ohms with 2 amps flowing = IR = 2 Amps * 2 Ohms = 4V.