Common Core State Standard: 8.EE.6 

Grade  8 
Subject  MATH 
Domain  Expressions & Equations 
Theme  Understand the connections between proportional relationships, lines, and linear equations 
Standard  Use similar triangles to explain why the slope m is the same between any two distinct points on a nonvertical line in the coordinate plane; derive the equation y =mx for a line through the origin and the equation y = mx + b for a line intercepting the vertical axis at b. 
Reference URL  http://www.corestandards.org/Math/Content/8/EE 