EXPLANATION
Replacing the given functions into the following:
[tex]\frac{f(x+h)-f(x)}{h}[/tex][tex]\frac{x^2-1+h-(x^2-1)}{h}[/tex]
Removing the parentheses and simplifying:
[tex]\frac{x^2-1+h-x^2+1}{h}[/tex]
Simplifying like terms:
[tex]\frac{h}{h}=1[/tex]
Hence,
[tex]\frac{f(x+h)-f(x)}{h}=1[/tex]
