We are asked to determine how many marbles must be in the seventh line but the condition stated in the problem must be followed such that "starting with a number in every consecutive line it has a number one more than the previous line".
We start with 3, followed by 3+1 = 4, followed by 4+1=5, followed by 5+1=6, followed by 6+1=7, followed by 7+1=8, followed by 8+1=9
The sequence is:
3,4,5,6,7,8,9
In the seventh, there are 9 total marbles.
