Consider the limit lim x->c (f(x)-f(c))/x-c. Let x=c+h
Rewrite and simplify, the limit above using the new expression for x.

a. lim h->0 (f(c+h) - f(c))/ h
b. lim h->0 (f(c+h) - f(c))/ c+h
c. lim h->0 (f(x) - f(c))/ h
d. lim h->0 (f(x) - f(c))/ x