The different lines that can be on a graph are vertical lines, horizontal lines, and oblique lines.
vertical
horizontal
oblique
vertical lines go along the y axis so the slope would be y/0 or “undefined” because you cant divide by 0.
horizontal lines go along the x axis so the slope for this line would be 0/x.
To determine if the line is negative or positive you need to look at the direction of the line and the height.
A line that would be negative would be starting on the left and goes higher on the right then the slope would be positive. But if the line is going from the left to right but going down then the slope would be negative.