Math Help
posted by Miguel .
Mobile ad spending. The amount spent worldwide in advertising on mobile devices can be estimated by f(x) =0.4x^2+0.01x+0.9. Where x is the number of years after 2005 and f(x) is in billions of dollars. When will worldwide mobile ad spending reaches $41 billion?

solve 41 = .4x^2 + .01x + .9
multiply by 100
40x^2 + x + 90 = 4100
40x^2 + x  4010 = 0
x = (1 ± √641601)/80 = 10 or a negative
it will take 10 yrs