Problems returning multiple columns

CREATE FUNCTION wklyavg(numeric, timestamp, timestamp) RETURNS TEXT AS '
DECLARE
system ALIAS FOR $1;
startdate ALIAS FOR $2;
enddate ALIAS FOR $3;
result numeric;

BEGIN
result := (select (avg(usr) + avg(sys)) from sardata
where systemid = system and time between startdate
and enddate);
return enddate || result;
END;
' LANGUAGE 'plpgsql';

Above is the function I'm trying to run and I keep getting errors telling
me I need to cast my values. I've played with it a bit but not knowing
much about casting I was hoping for some help. I'm basically just summing
a couple of averages for values between two given dates. I wanted to
return the summed average and the "enddate" as one row. Any suggestions?

Thanks,

Travis