md5 encryption and decryption

Question

            MD5 md5a1 = System.Security.Cryptography.MD5.Create();

            byte[] inputBytesa1 = System.Text.Encoding.ASCII.GetBytes(Part11);

            byte[] hasha1 = md5a1.ComputeHash(inputBytesa1);


this is my code for md5 encryption.. hoe to decrypt that..


please help me anyone..