y(x) = (5-x)^4 at x = 4

how do I solve this equation with the instantaneous rate of change formula ? any help would be greatly appreciated .I know I use f'(x)=lim{f((x+h)-f(x))/h) but the polynomial is giving me trouble ....please help ..the answer is -4 from a quiz I did but I got it wrong .

Bee