Let $\displaystyle {S}$ be the universal set, where:
$\displaystyle {S}={\left\lbrace{1},{2},{3},\ldots,{18},{19},{20}\right\rbrace}$
Let sets $\displaystyle {A}$ and $\displaystyle {B}$ be subsets of $\displaystyle {S}$, where:

Set $\displaystyle {A}={\left\lbrace{3},{4},{7},{11},{16},{17},{19}\right\rbrace}$

Set $\displaystyle {B}={\left\lbrace{1},{2},{3},{4},{9},{12},{15},{17},{18},{20}\right\rbrace}$

Find the following:

LIST the elements in the set ($\displaystyle {A}\cup{B}$):
$\displaystyle {\left({A}\cup{B}\right)}$ = { }
Enter the elements as a list, separated by commas. If the result is the empty set, enter DNE

LIST the elements in the set ($\displaystyle {A}\cap{B}$):
$\displaystyle {\left({A}\cap{B}\right)}$ = { }
Enter the elements as a list, separated by commas. If the result is the empty set, enter DNE

You may want to draw a Venn Diagram to help answer this question.