Help with thevenins voltage
You need to calculate the equivalent Thevenin voltage source.

To do that, you have to assume no current goes out of you circuit, i. e., no current flowing through the 7kΩ resistor. 

The bottom left part is equivalent to 48V source and 12kΩ resistance; the top right part is equivalent to 24V source and 4kΩ resistance. 

When you connect these two sides in parallel, the voltage will be somewhere between 24V and 48V. You have a voltage divider formed by 12kΩ connected to 48V and 4kΩ connected to 24V. The voltage difference is 48V-24V=24V. The voltage drop over the 4kΩ resistance will be 24V*4kΩ/(12kΩ+4kΩ)=6V. So the output voltage will be 24V+6V=30V, as you were suspecting.