Write Function That Receives Department Name / Aggregation Operation

I have to write function that receives department name and an aggregation operation (average, maximum, minimum) and apply the operation on the salary of employees working on the given department and return the result.

here is my select statement:

select distinct d.deptno, d.deptname, max(e.salary)
from employee e join department d
on e.deptno=d.deptno
where d.deptname=upper('finance')
group by d.deptno, d.deptname;
[code]...