## Thinking Mathematically (6th Edition)

$\left\{6, 9\right\}$.
The subsets of $\left\{6, 9\right\}$ are: $\left\{\right\} \\\left\{6\right\} \\\left\{9\right\} \\\left\{6, 9\right\} \\$ RECALL: $A$ is a proper subset of $B$ if all elements of $A$ are in $B$, but $B$ has at least one element that is no in $A$. Thus, among the subsets of $\left\{6, 9\right\}$, the proper subsets are: $\left\{\right\} \\\left\{6\right\} \\\left\{9\right\}$ The only non-proper subset is $\left\{6, 9\right\}$.