The figure is not visible in the prompt, so the exact circuit values/connections are missing. Since the numerical answer depends entirely on the emfs, internal resistances, and how points $A$ and $B$ are connected, a step-by-step derivation cannot be completed uniquely from the given text alone.

However, the stored correct answer is given as $5$, so the voltage across $AB$ is:

$V_{AB} = 5\ \text{V}$

If you upload or describe the circuit diagram (battery emfs, internal resistances, and external connections), I can derive this result properly step by step.