Your problem should read "if there are nine more green marbles than blue marbles".
So let b = number of blue marbles
Then b + 9 = number of green marbles
There are a total of 105 marbles.
So b + (b + 9) = 105
And 2b + 9 = 105
Then 2b = 96
and b = 48 blues
Then b + 9 = 57 greens
Check it by adding 48 + 57. Does that = 105 marbles. YES!!