Vector Calculus

$\nabla \phi = 2y\hat{i} + 2x\hat{j} - 2z\hat{k} = -2\hat{i} + 4\hat{j} - 2\hat{k}$. Unit vector $\hat{u} = \frac{3\hat{i} - 4\hat{k}}{5}$. $\nabla \phi \cdot \hat{u} = \frac{(-2)(3) + (4)(0) + (-2)(-4)}{5} = \frac{-6 + 8}{5} = \frac{2}{5} = 0.4$. (Taking +0.4).