Linear Graph Question

Question

So I have a linear graph:

By the graph itself, we can see it decreasing. But if we take these points: $(0,2)$ and $(4,0)$ we get $\frac{1}{2}$. I thought you can take any $2$ points on a linear graph to get a correct slope, why is it in this case not accurate? Obviously we can take $(0,2)$ and $(2,1)$ and get to the correct answer.

Answer 1

$\displaystyle\frac{0-2}{4-0}=-\frac12$, not $\dfrac12$.