topological order: s t y x z

take s (d=0)
    relax (s,t) w=6: 0+6=6 vs d[t]=INF -> improved
    relax (s,y) w=7: 0+7=7 vs d[y]=INF -> improved
take t (d=6)
    relax (t,x) w=5: 6+5=11 vs d[x]=INF -> improved
    relax (t,y) w=8: 6+8=14 vs d[y]=7 -> no change
    relax (t,z) w=-4: 6+(-4)=2 vs d[z]=INF -> improved
take y (d=7)
    relax (y,x) w=-3: 7+(-3)=4 vs d[x]=11 -> improved
    relax (y,z) w=9: 7+9=16 vs d[z]=2 -> no change
take x (d=4)
take z (d=2)

vertex |  d  | pi | shortest path from s
-------+-----+----+----------------------
   s   |   0 |  - | s
   t   |   6 |  s | s -> t
   x   |   4 |  y | s -> y -> x
   y   |   7 |  s | s -> y
   z   |   2 |  t | s -> t -> z
