What Is Vo in a Power Source Diagram Circuit?

[rh23]

Circuit help

Homework Statement

I have a book problem that show this pic (see attachment)
The problem wants me to calculate Vo and the end point of the graph and midpoint but I am not sure what Vo is.

Homework Equations

Vo=Va-Vb

The Attempt at a Solution

I think Vo is the total voltage of the power source so let say at the left end point of the graph we have (-4,-3) so because this is series i would say the total voltage was -7 for this point

 

[TaxOnFear]

rh23,
You are correct, V_o is sum of voltages across R and X.