Why does changing the x value make the the line move up or down?

Why does changing the x value make the the line move up or down?

References

Reference NO 1

http://www.khanacademy.org/math/algebra/linear-equations-and-inequalitie/slope-and-intercepts/v/slope-of-a-line

Reference NO 2

http://www.purplemath.com/modules/grphquad2.htm

Reference NO 3

http://www.enotes.com/homework-help/how-do-you-solve-graph-simple-y-mx-b-equation-61631

Reference NO 4

http://jwilson.coe.uga.edu/EMT668/EMAT6680.Folders/Barron/unit/Lesson%206/6.html

Reference NO 5

http://www.wtamu.edu/academic/anns/mps/math/mathlab/col_algebra/col_alg_tut27_graphline.htm

Reference NO 6

http://www.themathpage.com/alg/slope-of-a-line.htm

Reference NO 7

http://pages.towson.edu/racasey/tools/GenChemExcel.htm

Reference NO 8

http://www.algebralab.org/lessons/lesson.aspx?file=Algebra_AbsoluteValueGraphing.xml