Vector Algebra

Let $x_0$ be the point of local maxima of $f(x) = \bar{a} \cdot (\bar{b} \times \bar{c})$ where $\bar{a} = x\hat{i} - 2\hat{j} + 3\hat{k}, \bar{b} = -2\hat{i} + x\hat{j} - \hat{k}$ and $\bar{c} = 7\hat{i} - 2\hat{j} + x\hat{k}$ then the value of $\bar{a} \cdot \bar{c}$ at $x = x_0$ is