The answer is
9n-2
d=9
1#=7
You get everything you need from factoring the last expression:

The left side is a difference of squares, and we get another difference of squares upon factoring. We end up with

Plug in everything you know and solve for
:

In total, Kay took 5+14+2=21exams. she got 80%-90% on 14 of them. the probably is 14/21 which is 2/3. Therefore, the probability of Kay getting 80%-90% on her next exam is 2/3 or 66.6%
So we can make an equation for this, being 7=1/3x, as that effectively says she can make 3 drawings per page.
Knowing this, we can multiply it by 3 to get x=21 (since if she can draw 3 drawings per page, we can imagine that if each drawing took a page she would need 3 times the pieces of paper), therefore she can make 21 drawings.