How to tell if a slope is negative or positive:

You always read your data and slopes left to right.

If a slope is positive, the starting point will be less than your following points and your slope will move in an upward motion (/)

If a slope is negative, the starting point will be greater than the following points and your slope will move in a downward motion (\)

If your slope is a perfectly vertical line your slope is unidentified

if your slope is a perfectly horizontal line your slope is always 0