Thevenin Equivalent With Two Voltage Sources
I think there was some confusion in the comments, so I'll write up an answer.

The circuit we're talking about is:







simulate this circuit &ndash; Schematic created using CircuitLab

Note that I've left out the load - it has nothing to do with your Thevenin equivalent.

Your goal is to make a circuit that looks like:







simulate this circuit

so you need to find two things: the equivalent resistance and the Thevenin voltage. You already found \$R_{eq} = 75 \Omega\$, so I won't go over that. 

The Thevenin voltage of a circuit is the same as the open circuit voltage: when you leave the load disconnected, \$V_{th} = V_{ab}\$. That means that all you need to do is find \$V_a\$ and \$V_b\$ with no load.

\$V_a\$ is easy - it's just 3 V - so the harder part is
$$
V_b 
= (3V + 9V) \frac{100 \Omega}{150\Omega + 150\Omega + 100\Omega}
= 3 V
$$ 
so
$$
V_{th}
= V_a - V_b
= 0 V
$$

That means the Thevenin equivalent of this circuit is just a 75 ohm resistor - there's no voltage across the terminals.