Given that f: l --> R is a coordinate function for l.

Prove that g: l --> R defined by g(P) = f(P) + c for some constant c is also a coordinate function for l.

My textbook defines coordinate function as one that is both one-to-one and "onto" such that PQ = |f(P) - f(Q)|

I already figured out how to prove the one-to-one, but I'm having issues with proving the "onto" part of it. In class, my professor just said it was a matter of "adding c and subtracting c," but I couldn't figure out what he meant by that. Any ideas?