Result of the play is a touchback.
10-4-6 defines the basic spot for this type of play. The basic spot is the 20 yard line.
This is a foul by the offense behind the basic spot, so we will enforce from the spot of the foul. Foul enforced in the end zone = safety.
If B-42 had blocked on the B-2, it would not be a safety, but B's ball at the B-1 after enforcement.