Why the other quadrants cannot be used for graphing real world situations ?

Ijust need these 2 questions answered (the second pic is just confirmation i'm not confident in that answer)

Which of the following statements would be sufficient to prove that parallelograms pqrs is a rectangle ?

Aline passes through the points (5,4) and (2.1). part a.) what is the slope of the line that passes through these points? show work. part b.) what is the equation of the line that passes through these points. show work. part c.) where does the line intercept on the x& y axis? show work