Hello,

Since gcd(7,15)=1, you can find y such that 7y=1 mod 15. After that, multiply the whole by 3 in order to get x.

Euclidian algorithm, to find y :

15=7x2+1 --> -7x2=1-15=1 mod 15 (remember, i told you you could add as many times as you want 15).

-2 mod 15=-2+15 mod 15=13 mod 15

Therefore, y=13

Multiplying by 3 : x=39=9 mod 15

6x = = 5 (mod 15)

gcd = 3 but 3 isn't a divisor of 5 therefore there are no solutions??

Hmmm what is "incongruent" ?x^2 = = 1 (mod8) I get 4 and 1 as two incongruent solutions??

1²=1 mod 8

whereas 4²=0 mod 8