Book Maths Puzzle Co-ordinate geometry

how do I solve this:

Equation for verticle line passing through (8,-4)

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Book Other Question

1) Sue is both the 50th best & the 50th worst student at her school. How many students attend her school ?

2) In a race from point X to point Y & back, Jack averages 30 miles per hour to
point Y & 10 miles per hour back to point X. Sandy averages 20 miles per hour in both directions. Between Jack & Sandy, who finished first ? A candy factory classifies its candies as (Carmel-5 types) (chocolate 7 types) ( Dark chocolate 10 types). A customer wants (4 types of Carmel) (5 types of Chocolate) (7 types of Dark Chocolate) and i need to know how many assortments are possible?