Answer
15 Volts
Work Step by Step
First of all, we need to find the current going through the 3 Ohm resistor. To do this, we first find the current going through the middle branch with no resistors in it. We find the current to be:
$I=(4)(.5)=2\ A$
Kirchhoff's junction rule states that the current entering a junction and the current leaving a junction are equal. Thus, since we see that there are $1+2=3\ A$ leaving the junction, it follows that there are 3 Amps going into the junction, meaning that 3 Amps of current are going through the 3 Ohm resistor.
Thus, we can find the voltage using Kirchhoff's Voltage Law, which states that the voltage around a complete path is 0.
$v_s=(3)(3)+(4)(1)+(2)(1)=\fbox{15 Volts}$
