Well if the student already finished 6 problems out of 15, there are 15-6 = 9 homework questions she still has to finish.
Still needs to complete: 9.
Already finished: 6
9 : 6.
Simplified: 3 : 2
This sequence is not a geometric.
It is because, common ratio = 6/3 = 2
9/6 = 1.5
As the ratio's are not same, it is not a G.P
Hope this helps!
-1 is the Y intercept for the graph
Just multiply the two, your answer should be 21.06