A circle $C_1$ has radius $2$ units and a circle $C_2$ has radius $3$ units. The distance between the centres of $C_1$ and $C_2$ is $7$ units. If two lines, one tangent to both circles and the other passing through the center of both circles, intersect at point $P$ which lies between the centers of $C_1$ and $C_2$, then the distance between $P$ and the centre of $C_2$ is