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Answer:
9
Step-by-step explanation:
Three sides are
Let other side be x
[tex] \rm \mapsto \: x + 4 + 2 + 6 = 21 \\ \rm \mapsto \: x + 6 + 6 = 21 \\ \rm \mapsto \: x + 12 = 21 \\ \rm \mapsto \: x = 21 - 12 \\ \rm \mapsto \: x = 9[/tex]