You have the value of Va-VG and VB-VG. What will be VA-VB then?
What do you find confusing? A circuit works with potential "differences". In your circuit, the source is of 10V. Now, all the voltages present in the circuit are due to that 10V source, right? There is always a reference point in a circuit, called 'ground'. All the voltages are measured w.r.t ground. The choice of ground is ours i.e. we can assume it at any point. Here, for the sake of convenience, ground is assumed at the -ve terminal of the 10V source. Now, when you say voltage across 100 ohm is 8V, it is the potential difference between ground and point A, right? And 2.5V is the potential difference between ground and B. Since ground is common to both the resistors, you can say, A is 8V above the ground potential and B is 2.5V above the ground potential. It's like saying building A is 8m tall and building B is 2.5m tall( building height is measured from common ground, isn't it?). So, differene in height between buildings A and B would be 5.5m, right? Similarly, voltage between A and B is 5.5V.The math makes sense, but the other stuff is still a bit confusing.
That's the part that kinda bugged me. I always think of voltage/potential across something or between 2 points. I wasn't sure why you can just say point A is at this potential and point B is at that potential. But that makes sense now. the potential in that portion of the wire is 8V so that means point A is at 8V right? Same for point B?Since ground is common to both the resistors, you can say, A is 8V above the ground potential and B is 2.5V above the ground potential.