I have to show that, $$\oint (\vec{K} \cdot \vec{r})d\vec{l}=-\vec{K} \times\int d \vec{a}$$
Here, $\vec{K}$ is constant vector and $\vec{r}$ is position vector.
I have approached by applying Stokes' Theorem,
$$\oint (\vec{K} \cdot \vec{r})d\vec{l}=\int \nabla \times (\vec{K} \cdot \vec{r}) d \vec{a}$$
At this point, I'm stuck and unable to draw any further results.