Question

solve for n - 21 = 3

Answer 1

Add 21 to both sides
n - 21 = 3
n - 21 + 21 = 3 + 21
n - 0 = 24
n = 24

Answer 2

Question

solve for n - 21 = 3

n - 21 = 3

n = 21 + 3

n = 24 (your answer)

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check

24 - 21 = 3

the answer is good