Trailhead Module 3:  Create a formula field that determines the number of days between today and the last activity date for a case's account.
Your support team has asked for improved visibility on account activity level at the time they’re helping with customer issues. Specifically, when they’re looking at a case, they’d like to see an at-a-glance view of the number of days since the case’s related account was last active. Create the formula using these requirements.The formula should be on the Case object.
The formula should be of return type Number.
The formula should be named 'Days Since Last Update' and have a resulting API Name of 'Days_Since_Last_Update__c'.
The formula should return the number of days between the account’s Last Activity Date and today.

I created the following:

I created a new custom formula field with return type number and called Days Since Last Update.
I created this formula: Account.LastActivityDate - TODAY()

I get this error message:

Challenge not yet complete... here's what's wrong: 
The 'Days_Since_Last_Update__c' formula field did not return the correct number of days between an Account’s Last Activity Date and today

Can you please assist what I did wrong?  I'm not sure if my formula is even correct with the challenge.  Please help!

Thank you.

This challenge seems simple enough but I'm stuck and any help would be appreciated. 

So the Challenge is Create a validation rule to check that a contact is in the zip code of its account. here is the question below:
To complete this challenge, add a validation rule which will block the insertion of a contact if the contact is related to an account and has a mailing postal code (which has the API Name MailingPostalCode) different from the account's shipping postal code (which has the API Name ShippingPostalCode).Name the validation rule 'Contact must be in Account ZIP Code'.
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.
The validation rule should ONLY apply to contact records with an associated account. Contact records with no associated parent account (hint: you can use the ISBLANK function for this check) can be added with any MailingPostalCode value

Here is my  work.. Any help would be appreciated.. thanks, 

Rule Name Contact_must_be_in_Account_ZIP_Code Active [Checked]
Error Condition Formula AND( BillingPostalCode = ShippingPostalCode )
Error Message Billing zipcode does not match the Shipping Zipcode Error Location Billing Zip/Postal Code