One way to calculate the slope of a line is by drawing a slope triangle. For example, using this slope triangle, the slope of the line is $\text-\frac24$, or $\text-\frac12$. The slope is negative because the line is decreasing from left to right.

Another way to calculate the slope of this line uses just the points $A:(1,5)$ and $B:(5,3)$. The slope is the vertical change divided by the horizontal change, or the change in the $y$-values divided by the change in the $x$-values. Between points $A$ and $B$, the $y$-value change is $3-5=\text-2$ and the $x$-value change is $5-1=4$. This means the slope is $\text-\frac24$, or $\text-\frac12$, which is the same value as the slope calculated using a slope triangle.

Notice that in each of the calculations, the value from point $A$ was subtracted from the value from point $B$. If it had been done the other way around, then the $y$-value change would have been $5-3=2$ and the $x$-value change would have been $1-5=\text-4$, which still gives a slope of $\text-\frac12$.