No wonder you cannot get the right answer, you got the greatest commond divisor all wrong.

Watch frum die meister und lern.

660=15*42+30

42=1*30+12

30=2*12+6

12=2*6+0

Thus, gcd(42,660)=6.

Now linear combinations:

30-2*12=6

30-2(42-30)

30-2*42+2*30=3*30-2*42=6

3(660-15*42)-2*42=6

3*660-47*42=6