Hi William,

Let's take a Look at the problem and then solve it by breaking into smaller components:

• A contact with a MailingPostalCode that has an account and does not match the associated Account ShippingPostalCode should return with a validation error and not be inserted.

           Contact may or may not have associated with the                              Account.Here we need to make sure that Contact has an                  associated Account.In other words,'Account Name' on the                Contact object should not be blank.To check this                              conditon,Salesforce has provided 'ISBLANK' Function:

           ISBLANK(expression)
          Checks whether an expression is blank and returns TRUE or            FALSE

           Account Name should be blank:

           ISBLANK(Account.Name)

           To make the above condition negative,we have 'NOT'                         Function:

            Account Name should not be blank:

            NOT(ISBLANK(Account.Name))

• A contact with a MailingPostalCode that has an account and does not match the associated Account ShippingPostalCode should return with a validation error and not be inserted.

           Account's ShippingPostalCode should not match Contact's              MailingPostalCode.Whenever this condition is true, we have            to give an error to the user.

           Here we get our second part of the formula:
           
           MailingPostalCode <> Account.ShippingPostalCode

           Now,We need to throw a validation error,whenever these two           conditions are true simultaneously.Here 'AND' Function                     comes very handy:

          AND(logical1,logical2,...)
         Checks whether all arguments are true and returns TRUE if all          arguments are true

         AND( 
              (NOT(ISBLANK(Account.Name))), 
              (MailingPostalCode <> Account.ShippingPostalCode) 
          )