I'm trying to show that $\sup \{ rx \ | \ x \in S \} = r \sup S$ where $S$ is a set of real numbers and $r$ is a real number greater than 0.
I started by letting $b = \sup S$. That would imply that $b \geq x$ for all $x \in S$ and $b \leq y$ for any upper bound $y$.
Since $b \geq x$, that implies $rb \geq rx$. So I've shown that $rb$ is an upper bound of $\{ rx \ | \ x \in S \}$.
Edit: show that $rb$ is the least upper bound of $\{ rx \ | \ x \in S \}$.
Since $b \leq y$, then $rb \leq ry$. This means that $rb$ is less than or equal to any upper bound of $\{rx \ | \ x \in S \}$. So $rb$ must be the least upper bound.