And the convention is is we measure the increase in vertical for a given in increase in horizontal. Find tan-1 -1 Type -1 and use the tan-1 key on a scientific calculator. This is very important.
For slope it tends to be kids putting it upside-down or messing up the negatives.
Have a play drag. The slope of a line in the plane containing the x and y axes is generally represented by the letter m and is defined as the change in the y coordinate divided by the corresponding change in the x coordinate between two distinct points on the line. A line with a positive slope increases if you look at it from left to right. Parallel to the x-axis.