Answer
This means that the number of phones with wifi will reach 150 million a little after the beginning of 2015.
Work Step by Step
We use the quadratic $c(x)=-.4x^2+21x+35$ to model the number of phone wifi-enabled cellphones, in millions. We are asked to find when the number will reach 150 million. This gives:
$$0=-.4x^2+21x+35-150$$
$$0=-.4x^2+21x-115$$
We use the quadratic formula
$$x= \frac{-b \pm \sqrt{b^2-4ac}}{2a}$$
Where b is the coefficient of the term with a variable raised to the first power, a is the coefficient of the squared term, and c is the coefficient of the term without a variable in front of it.
We only consider the answer that makes sense in the context of the problem:
x=6.25
This means that the number of phones with wifi will reach 150 million a little after the beginning of 2015.
