Bella eats during the week, 6 apples/day * 7 days/week = <<6*7=42>>42 apples/week
Grace harvests 42 apples/week * 3 = <<42*3=126>>126 apples during the week.
Leftover after Bella eats, 126 apples/week - 42 apples/week = <<126-42=84>>84 apples/week.
After 6 weeks Grace will have 84 apples/week * 6 weeks = <<84*6=504>>504 apples.
#### 504