hmmmm well, it turns out that 8x + 4, is a linear function, and thus its slope is a constant, you don't quite need to get the derivative of it, since it's already in slope-intercept form and we know the slope is "8" anyway.
but its derivative will be [tex]\bf f(x)=8x+4\implies \left. \cfrac{dy}{dx}=8\right|_{x=9}\implies 8[/tex]
