Your answer is -43/6
1/12(4+6x3)-9
=1/12(22)-9
=11/6-9
=11/6-54/6
=-43/6
thousands period is the answer
y = -1/2x + 5 should be your equation because it is 5 ft above sea level and its eroding, or going down, 1 ft per 20 years. So every year, the sand dune erodes 1/20 ft more.
Hope this helps!
