What is the slope of this line?

A line is increasing if it goes up from left to right. The slope is positive, i.e. m > 0 {\displaystyle m>0} .

A line is decreasing if it goes down from left to right. The slope is negative, i.e. m < 0 {\displaystyle m<0} .

If a line is horizontal the slope is zero. This is a constant function.

If a line is vertical the slope is undefined (see below).