2014 - 2006 = 8 years
8,600 - 3,800 =$4,800 - Growth over 8 years
$4,800 / 8 = $600 - Linear growth per year.
2014 - 2002 = 12 years since the initial deposit.
12 x $600 = $7,200 - growth over 12-year period.
$8,600 - $7,200 = $1,400 - initial deposit in 2002.
So that the linear function would look like this:
$8,600 =$1,400 + $600n, where n is number of years since 2002.