( 4 3 | 12 )
( 6 -5 | 2 )
5R1:
( 20 15 | 60 )
( 6 -5 | 2 )
3R2:
( 20 15 | 60 )
( 18 -15 | 6 )
R1+R2:
( 38 0 | 66 )
( 18 -15 | 6 )
R1/38 and R3/3:
( 1 0 | 33/19 )
( 6 -5 | 2 )
R3-6R1:
( 1 0 | 33/19 )
( 0 -5 | -160/19 )
R3/(-5):
( 1 0 | 33/19 )
( 0 1 | 32/19 )
a=33/19, b=32/19.