18,995 × 0.82^(6) = $5,774 so he should not sell
if the percentage decrease is 18% then do 100% - 18% which is 82. divide by 100 = 0.82. if it does this for 6 years then do 18,995 × 0.82^(6) because it's 6 years. the number is larger than Manuel's offer therefore he shouldnt sell