I'm trying to compare number of business hours with number of hours (bh<48) I have gone through few methods and tried the below method to get number of hours(difference between startdate and enddate :

diff(BusinessHours.id, startdate, Datetime.now())<48)

But I'm constantly getting the same error for all the methods : "Method does not exist or incorrect signature: [BusinessHours].diff(Schema.SObjectField, Datetime, Datetime)"

I dont understand this error as usage of methods are referred here

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_businesshours.htm

Any suggestions are highly appreciated! TIA

//Trigger to compare business hours with specific number of hours in apex trigger and update the case status value
    Trigger Test_Business_hours_Trigger on emailmessage (before insert,after insert,before update,after update) {
       // case cas {get;set;}
       long l;

    Case cas = [SELECT ClosedDate, status FROM Case WHERE ClosedDate != NULL LIMIT 1];
//SOQL query to get the business hours defined in the org 
       BusinessHours bh =
           [Select
             id,SundayStartTime, MondayStartTime, TuesdayStartTime,
             WednesdayStartTime, ThursdayStartTime, FridayStartTime,
             SaturdayStartTime, SundayEndTime, MondayEndTime,TuesdayEndTime,
             WednesdayEndTime, ThursdayEndTime, FridayEndTime,SaturdayEndTime
           From BusinessHours 
          Where IsDefault=true];
//gets closed date and system.now() in to the variables to define in business hours bh.diff() method
        DateTime startdate = cas.ClosedDate ;
        DateTime systemnow = system.now();
    
        long startl = startdate.getTime();
        long nowl   = systemnow.getTime();
// assigning the number of hours(business hours) i.e, the difference between the startdate and systemnow in to variable 'l'
        l = businesshours.diff(bh.id,startdate,systemnow);
        system.debug(l);
//For trigger.new values if status is closed and number of business hours are less than 48 hours(172800000 milliseconds) then updates the status to working
For(Emailmessage em : trigger.new){
        if((Cas.Status=='Closed') && (l<172800000)) {
            cas.status='Working';
            update Cas;
        }
}
    }