Answer
$^{39}_{18}Ar$
Work Step by Step
Recall, the superscript gives the number of protons plus the number of neutrons, so add the number of protons and the number of neutrons to get this number. The subscript, contrarily, indicates the number of protons present. We find the number for the superscript and then complete the entire symbol.
Mass number = 18 protons + 21 Neutrons = 39
This gives:
$^{39}_{18}Ar$