How do you estimate a family spending on food if you assume that the spending on food increases linearly and a family spent $3600 on food last year and$3000 the year before the last?

If the family spent $3000 two years ago, and$3600 one year ago, the growth between those years was:
$3600 -$3000 = $600 If food spending increases linearly, then the growth from one year ago to this year would once again be the same amount,$600.
$3600 +$600 = \$4200