Answer
$v(t)=-20t+40\quad $(billion per year)
Work Step by Step
We are given the 'acceleration' of debt (which is $v'(t)$)
$a(t)=-20 \quad$(billion per year) per year
Then,
$v(t)=\displaystyle \int(-20)dt=-20t+C$
We are also given that at $t=0$ (the start of 2008)
$v(0)=40 \quad$(billion per year)
$40=-0+C$
$C=40$
so,
$v(t)=-20t+40\quad $(billion per year)
