Thevenin theorem about Vth
Since you have replaced your \$6k\Omega\$ resistor for calculating \$R_{TH}\$, I am assuming that you want to calculate the \$V_{TH}\$ across the same terminals.  

Now what you have to calculate is the voltage across the open-circuit terminals. That can be calculated as \$V_a - V_{2k\Omega}\$. Where \$V_{2k\Omega}\$ is the voltage drop across \$2k\Omega\$ resistor, to calculate which you need to know the current through it. If you can replace the four resistors to the left of the voltage source with an equivalent resistance, then the current through the \$2k\Omega\$ resistor and hence \$V_{TH}\$ can be calculated easily.