9x ≡ 12 (mod 21)

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Q : 9x ≡ 12 (mod 21)

A :

9 | 12 + 21k ( k is a possitive integer )
3 | 4 + 7k

k (min.) = 2 ,

x = (12 + 21 × 2) ÷ 9 = 6

Answer : x = 6 (mod7)

9x ≡ 12 (mod 21)

3x ≡ 4 (mod 7)

x ≡ 6 (mod 7)

9x = 12

9x = 12
9 9

x = 1 1
3