Answer
$a)$ $f(x+h)=1-x-h$
$b)$ $f(x+h)-f(x)=-h$
$c)$ $\frac{f(x+h)-f(x)}{h}=-1$
Work Step by Step
$a)$
$$f(x)=1-x$$
Replace $x$ with $x+h$:
$$f(x+h)=1-(x+h)$$
$$f(x+h)=1-x-h$$
$b)$
$$f(x+h)-f(x)$$
Substitute corresponding expressions:
$$1-x-h-(1-x)$$
$$1-x-h-1+x=-h$$
$c)$
$$\frac{f(x+h)-f(x)}{h}$$
Substitute corresponding expression for numerator:
$$\frac{-h}{h}=-1$$