Show that if Ax=b has more than 1 solution, it has infinitly many solutions.

Hint: If $\displaystyle u_1$ and $\displaystyle u_2$ are two solutions consider $\displaystyle w=ru_1+su_2$ where $\displaystyle r+s=1$

Here's my start

let $\displaystyle u_1$ and $\displaystyle u_2$ be distinct solutions to $\displaystyle Ax=b$ then $\displaystyle Au_1=b$ and $\displaystyle Au_2=b$

Why must r+s=1?