Hi I am currently progressing on developer trailhead and i am currently on Formulas and Validations >> Using Formula Fields. I am not able to identify what exactly i am missing here. The steps to be followed are
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 have created a custom obj Case and Added a Formula field with the name as mentioned. Formula i have used in Today () - LastActivityDate . But when i check my activity it display error "The 'Days_Since_Last_Update__c' does not exist "..

My first confision in the steps is "The formula should return the number of days between the account’s Last Activity Date and today.". what does ACCOUNT'S Last Activity Date means here .. do i need to somehow relate Case and Accounts object first  ?

Any help/suggestions please ?