A function is executed like any other SQL built-in function:
SQL> SQL> create or replace function calculate_area 2 (v_length in number, v_width in number) 3 return number 4 is 5 v_area number; 6 begin 7 v_area := v_length * v_width; 8 return v_area; 9 end; 10 / Function created. SQL> SQL>