If the pattern below follows the rule "starting with three, every consecutive line has a number one more than the previous line," how many marbles must be in the seventh line?

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.