I'm sure the proof is really easy, but I am relatively new to these proofs and I'm not really sure how to start...

Let f: R $\displaystyle \rightarrow$ R and let c $\displaystyle \in$ R. Show lim as x $\displaystyle \rightarrow$ c of f(x) = L if and only if lim x $\displaystyle \rightarrow$ 0 of f(x+c) = L

(sorry about the code, this is the first time I have used Latex...can anyone suggest an all in one reference for Latex coding?)