Question

`6+x=21`
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Answer 1

15

Explanation:

`6 + x = 21 \\ \\ x = 21 - 6 \\ \\ x = 15`

Hope it help you

Answer 2

21 - 6 = x

so the answers is 15

21 - 6 = 15

check:
15 + 6 = 21