How do you determine if a slope is positive or negative?

As you look at the graph from left to right (increasing x), if the line is going up(increasing y), slope is positive.

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If it's going down (decreasing y), slope is negative.

If the slope in the graph is positive, the line starts from bottom left towards the top right.
If the slope in the graph is negative, the line starts from the upper left to the bottom right.