Increase or Decrease
A line with a positive slope points up to the right.  A line with a negative slope points down to the right.

A linear function is increasing when the \(x\)values increase and the \(y\)values increase.
This line is increasing. An increasing line has a positive slope \((m>0)\). 
A linear function is decreasing when the \(x\)values increase and the \(y\)values decrease.
This line is decreasing. A decreasing line has a negative slope \((m<0)\). 
A linear function is constant when the \(x\)values increase and the \(y\)values remain the same.
This line is a horizontal line. A horizontal line has zero slope \((m=0)\). 
A linear function is undefined if the \(x\)values are constant as the \(y\)values vary.
This line is a vertical line. A vertical line has undefined slope. 