Difference

Consider the following two events #A# and #B#:
\[\begin{array}{rcl}
A &=& \{5,\,13,\,18,\,19,\,20,\,21\}\\
B &=&\{3,\,5,\,11,\,14,\,18,\,21\}
\end{array}\]

What is the difference of #A# and #B#?

Note: Enter the elements in order from smallest to largest.

#A \,\backslash\, B = \{#, , #\}#