Why can’t we consider points in second quadrant as i can be anywhere. Also what is the purpose of 45 degree?
It can’t be anywhere, coordinate geometry is to scale in the GRE.
It’s to tell you that it’s a y-x line. Since it would form a 90-45-45 triangle at any point, so the height and width of the triangle (x and y) would be the same.
The purpose of 45 degree is to tell that for every point on the line K, y=x. Keeping that in mind, if the desired point (m,n) lies above line K , n>m. you can figure this out once you try to map out any point above K.