Question

f(x)=0.485x^2-1.694x+0.135, where the output is in millions of users. In this formula x=6 corresponds to 1996, x=7 corresponds to 1997, and so on until x=20 corresponds to 2010 estimate when the number of internet users in the country reached 85 million

Answer 1

I reckon you have to find x, when f(x) = 85. In other terms:
\[85 = 0.485x^2 - 1.694x + 0.135\]You can probably solve for x, using the quadratic formula:
\[x=\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\]

Answer 2

Where \[ax^2+bx+c=0\]