To find the slope of a line, you first chooses 2 points with known coordinates that belongs to the line. Let those points be A(x1,y1) and B(x2,y2) with x2>x1.
You then divide the subtraction of their Ys as y2-y1 by the subtractions of their Xs as x2-x1 (the point with the higher x comes first, this is why x2>x1). And let the slope be m.
m= (y2-y1) / (x2-x1). If the slope is positive, the line croissant and y2>y1. If the slope is negative, the line is decroissant and y1>y2.
Hope this helps! :)
