Here they are deriving the equation for Young's experiment: http://www.physicsclassroom.com/Class/light/U12L3c.cfm
The part where they start bringing up "Assertion" and "Logic/Rationale" is where I got confused:
They mention for part ii) that if the distance L >>> y, then S1P is || (parallel) to BP.
If they are parallel to each other, wouldn't the two lines NOT connect together to form a triangle? What I know about parallel lines, they never intersect, and in this case, they are intersecting at P.iii
S1P = BP
If S1P and BP are || and line S1B is perpendicular to BP, then the length BP = length S1P.
Young's Equation for a double slit experiment