Answer
See below
Work Step by Step
The year 1980 is $20$ years after the year 1960; substitute $x=20$:
$y=-0.0006(20)^3+0.0383(20)^2+0.383(20)+68.6\\=-0.0006(8000)+0.0383(400)+0.383(20)+68.6\\=-4.8+15.32+7.66+68.6\\=86.78$
In the year 1980, $86.78$ million people voted.
The year 1960 is $0$ years after the year 1960; substitute $x=0$:
$y=-0.0006(0)^3+0.0383(0)^2+0.383(0)+68.6\\=-0.0006(0)+0.0383(0)+0.383(0)+68.6\\=0+68.6\\=68.6$
In the year 1960, $68.6$ million people voted.
The difference between the number of voters: $86.78-68.8=18.18$ people
