You’re correct, it’s 16. Constant proportionality mean how much Y moves. So it’s 16!
A(n) = –3 • 2⁽ⁿ⁻¹⁾
for n = 1 , A₁ = -3.(2)⁰ = -3
for n = 2 , A₂ = -3.(2)¹ = -6
for n = 3 , A₃ = -3.(2)² = -12
for n = 4 , A₄ = -3.(2)³ = -24
...........................................
for n = 8 , A₈ = -3.(2)⁷ = -384
