By 2007, cellphones with 2-gigabyte storage cards could hold 2000 songs. The bar graph sho...

By 2007, cellphones with 2-gigabyte storage cards could hold 2000 songs. The bar graph shows the percentage of new cellphones that played music from 2005 through 2007, with projections for 2008 and 2009.

The formula

M = −1.8x2 + 21x + 15

models the percentage of new cellphones that play music, M, x years after 2005.

a. Use the formula to find the percentage of new cellphones that will play music in 2009. Does this overestimate or underestimate the percentage shown by the graph? By how much?

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b. Use the formula to determine the first year in which 75 % of new cellphones will play music.