### Particle dynamics, Newton laws

A body (mass $m$) is thrown horizontally with the initial velocity $\vec {v}_0$. The air friction force is $\vec {F}_f=-k\vec {v}$, where $k=\text{const}$ and $\vec {v}$ is the body velocity. Find $\vec {v}(t)$ and $\vec {r}(t)$.