Difference

Consider the following two events #A# and #B#:
\[\begin{array}{rcl}
A &=& \{3,\,4,\,10,\,18,\,19,\,20\}\\
B &=&\{3,\,10,\,13,\,17,\,18,\,21\}
\end{array}\]

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

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

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