3. (2 points) The force exerted by an electric charge at the origin on a charged particle at a point \\
$(x, y, z)$ with position vector $\vec{r} = (x, y, z)$ is $\vec{E}(\vec{r}) = K \frac{\vec{r}}{|\vec{r}|^3}$ where $K$ is a constant.\\
(A) Find a scalar potential function for $\vec{E}$.\\
(B) Find the work done as the particle moves along a straight line from $(-1, 9, 0)$ to $(2, 6, 7)$.