Using Thevenin theorem, find the equivalent Thevenin circuit for the highlighted circuit from the a-b terminals.
Solving for $R_{TH}$
$$\begin{align}
R_{TH} &= R_1 + R_2 + R_3\\
&= 60\Omega + 20\Omega + 50\Omega \\
&= 130\Omega
\end{align}
$$
Solving for $E_{TH}$
$$\begin{align}
E_{TH} &= -E_1 -R_2\cdot I_2\\
&= -8V - (20\Omega) \cdot (150 mA)\\
&= -8V - 3V\\
&= -11V
\end{align}
$$