I have two points, $p\_1$ and $p\_2$, in a cartesian plane, and a fixed radius, $r$.

I want to find the coordinates of another point, $p\_3$, that is in the same line of the $p\_1$ and $p\_2$, and always in a fixed distance, $r$, from the point $p\_1$.

$(a,b)$       $(?,?)$                      $\quad \quad \quad\space(c,d)$  

$p\_1$---------$p\_3$---------------------$p\_2$

        $\quad r$ 

Considering that the points $p\_1$ and $p\_2$ can be on anywhere in the plane.

Thanks in advance.