Field Validation for Item field on Service Request

Hello,

I am trying to set up a field validation for the service request form. The requirement is that: if Warranty Request = 'Y' and Item Number is not null then the Quantity field must be greater than '0'. I created a custom field called 'Item Validation' with a default value of Timestamp (). The intent is to set the validation up on this field. Any suggestion on the syntax to use for the validation expression?

Thank you.