I'm just a bit confused as to how to prove this.

4. (4 marks) A metric space (M, d) is separable if admits a countable subset D which is dense in M. Prove that every separable metric space (M, d) embeds isometrically into (50, \"\"00) That is, if (M, d) is separable, construct an isometry z' : (M, d) > (3\