Answer
$d(n)=32n-16$
Work Step by Step
From part (a), we have:
$d(n)=D(n)-D(n-1)$
Thus, we have
$d(n)=16n^2 -16(n-1)^2$
When we plug in $n=1,2,3,4$:
This gives: $16, 48,80,112$
$d(n)=16n^2 -16(n^2-2n+1)$
Thus, $d(n)=32n-16$