Answer
1) The open interval is $(3-\frac{c}{m},3+\frac{c}{m})$
2) $\delta=\frac{c}{m}$
Work Step by Step
Find a $\delta\gt0$ such that for all $x$ $$0\lt |x-3|\lt\delta\Rightarrow|mx-3m|\lt c\hspace{1cm}m\gt0, c\gt0$$
1) Find the interval around $3$ on which $|mx-3m|\lt c$ holds.
Solve the inequality: $$|mx-3m|\lt c$$ $$-c\lt mx-3m\lt c\hspace{1cm}(c\gt0)$$ $$-\frac{c}{m}\lt x-3\lt\frac{c}{m}\hspace{1cm}(m\gt0)$$ $$3-\frac{c}{m}\lt x\lt 3+\frac{c}{m}$$
The open interval around $3$ is $(3-\frac{c}{m},3+\frac{c}{m})$.
2) Give a value for $\delta$
Both endpoints are equally distant from $3$, the distance of which is $\frac{c}{m}$
So if we take $\delta=\frac{c}{m}$ (since $m\gt0$ and $c\gt0$, $\delta=\frac{c}{m}\gt0$) or any smaller positive number, then $0\lt|x-3|\lt\frac{c}{m}$, meaning all $x$ would be placed in the interval $(3-\frac{c}{m},3+\frac{c}{m})$ so that $|mx-3m|\lt c$.
In other words, $$0\lt |x-3|\lt\frac{c}{m}\Rightarrow|mx-3m|\lt c$$
