Y = kx
Plug in what we know:
5 = k(8)
5 = 8k
Divide 8 to both sides:
k = 0.625
Plug this back into the equation along with y = 15:
y = kx
15 = 0.625x
Divide 0.625 to both sides:
x = 24
Does it have to be a different number if not do nine cause then the bottom line soul be 6 and the sides would be 9
For this case we propose a rule of three:
31700 ------------> 8.8%
x ----------------------> 100%
Where "x" represents the number of copies sold to date.

Thus, to date 360,227 copies of the book have been sold.
ANswer:
To date, 360,227 copies of the book have been sold.
Y=mx+b, m=(y2-y1)/(x2-x1)
m=(7-4)/(6--3)=3/9=1/3
y=x/3 +b, now use either point to solve for b, I'll use (6,7)
7=6/3 +b
7=2+b
b=5 so our line is:
y=x/3 +5 or more neatly...
y=(x+15)/3
4x^2+4x-8=0
X^2+x-2=0
(X+2)(x-1)=0
X= -2 or 1