What are the 3 solutions? I'm stuck! 6x=15(mod 21)

Question

a=6,m=21,b=15 d=gcd(6,21)=3 solns.
6x=15(mod 21) 2x=5(mod 7)
21=6(3)+3
6+3(2)+0
0=6-6
6-(21-6(3))=3
6-21+18=3
6( )-21()=3

(21-6(3))(15)-6=3 ???????

Answer 1

6x = 21y + 15 for some y.
or,

2x = 7y + 5
since 2x is even, y must be odd.

y x
1 6
3 13
5 20
7 27
. . .
any number x of the form 2x = 7(2k+1)+5 is a solution, since

2x = 14k+7+5 = 14k+12
6x = 42k + 36
and 36 = 15 mod 21