a) Positive slope.
All the lines below have a positive slope
The upper tip of each line is inclined towards the positive x-axis direction.
b) Negative slope
All the lines below have a negative slope
The upper tip of each line is inclined towards the negative x-axis direction.
c) No slope (slope equal to zero)
Due to the equation for the slope of a line
A slope equal to zero implies that y_2=y_1, this is a horizontal line
d) Undefined slope.
This kind of slope is a consequence of x_2=x_1 in the equation above. On the plane, this slope corresponds to a vertical line