The order of these is 3,1,6,2,5,4
You can tell that 3 is the youngest because it's an intrusion and it's pushed its way to the surface.
4 is the oldest because it goes the farthest down out of each layer.
2 and 3 are intrusions so they are going to be young.
1 is also very young because it is at the top of the layers.
5 and 6 are just there they are both in the middle of young and old.
As you can see where b is labeled there is a crack in the layers there is actually an intrusion on both sides of b so they are the same.