It's one of the most-asked questions:
"When I'm 6-4 and open the 6-card suit, should I repeat the 6-card suit or bid the 4-card suit next?"
Bad news -- there is no "right answer." There is no definitive rule or agreement among teachers and experts.
Here are some mainstream thoughts/guidelines:
With 6 of a minor and 4 of a major
...
6 
A98 
QJ75 
AJ1062