SQL & PL/SQL

I have a two object types:

create or replace type figure as object(
name varchar2(30),
member function perimeter return varchar2) not final;

create or replace type body figure as
member function perimeter return varchar2 is    --varchar2 return type
begin
  return 'unable to calculate';
  end;
end;

create or replace type triangle as object(
side_1 number,
side_2 number,
side_3 number,
overriding member function perimeter return number);

create or replace type body triangle as
overriding member function perimeter return number is    --number return type
begin
  return self.side_1 + self.side_2 + self.side_3;
  end;
end;

I know that it throws error because I have different return types. But how can I implement my intentions?

Well basically my university ask me to implement some member method overloading maybe anyone can write me a little example or something?