Hello!  I'm working through the App Builder Trail and I've reached a point where I'm stuck and I was hoping someone could help me out here.  I'm currently working on the Formulas & Validations Module in the Create Validation Rules unit.

I believe the point where I'm hung up is:

"The validation rule should ONLY apply to contact records with an associated account. Contact records with no associated parent account can be added with any MailingPostalCode value. (Hint: you can use the ISBLANKfunction for this check)"

I'm new to creating formulas and validation rules.  I've tried inserting several different combinations if fields/operators/ and functions and I'm still stuck.  Any help would be appreciated.