The slope is defined as
[tex] m = \dfrac{\Delta y}{\Delta x} [/tex]
Where [tex] \Delta x,\ \Delta y [/tex] are the variations of the x and y coordinates of the two points.
Plugging your values, we have
[tex] \dfrac{8-0}{x-(-5)} = -2 \iff \dfrac{8}{x+5} = -2 \iff 8 = -2x-10 \iff 2x = -18 \iff x = -9 [/tex]
