Hey, I am new in apex. I am having an error of Variable does not exist: fflib_SObjectDomain which I have no idea how to fix it. 

Below is my code:

public class Accounts extends fflib_SObjectDomain{
    public Account (List<Account> accounts){
        super(accounts);
    }
      public class Constructor implements      fflib_SObjectDomain.IConstructable {
        public fflib_SObjectDomain construct(List<SObject>sObjectList) {
            return new Accounts(sObjectList);
        }
    }
}

Error message: Variable does not exist: fflib_SObjectDomain

Can anyone help me out?

Hey, I am new in apex. I am having an error of Variable does not exist: fflib_SObjectDomain which I have no idea how to fix it. 

Below is my code:

public class Accounts extends fflib_SObjectDomain{
    public Account (List<Account> accounts){
        super(accounts);
    }
      public class Constructor implements      fflib_SObjectDomain.IConstructable {
        public fflib_SObjectDomain construct(List<SObject>sObjectList) {
            return new Accounts(sObjectList);
        }
    }
}

Error message: Variable does not exist: fflib_SObjectDomain

Can anyone help me out?