Constructive interference occurs when 2 crests (white arcs) or 2 troughs (black arcs) meet, resulting in a bright fringe.
Destructive interference occurs when a crest (white arc) and a trough (black arc) meet, resulting in a dark fringe.
Distance between any 2 bright fringes x = λD/d
where λ = wavelight of light, D = distance from slits to screen, d = distance between the 2 slits.