Two cells \(\varepsilon_1\) and \(\varepsilon_2\) are connected in opposition to each other as shown in the figure. The cell \(\varepsilon_1\) is of emf \(9\ \text{V}\) and internal resistance \(3\ \Omega\). The cell \(\varepsilon_2\) is of emf \(7\ \text{V}\) and internal resistance \(7\ \Omega\). The potential difference between the points \(A\) and \(B\) is: