G7.2 $M, N$ are the points $(3,2)$ and $(9,5)$ respectively.
If $P(s, t)$ is a point on $M N$ such that $M P: P N=4: r$, find $s$.
$\begin{array}{l}M P: P N=4: 2=2: 1 \\ s=\frac{3 \times 1+9 \times 2}{2+1}=7\end{array}$