$ \textbf{Answer:}$
$-3$
$ \textbf{Explanation:}$
We are given $ f(x) = -3x+4$ which implies $ f(x+h) = -3(x+h)+4$
so,
$ \frac{f(x+h) - f(x)}{h}$
$ \implies \frac{ (-3(x+h)+4) - (-3x+4)}{h}$
$ \implies \frac{ -3x-3h+4+3x-4}{h}$
$ \implies \frac{-3h}{h} $
$ \implies -3$