Answer
${{\$}} 175$ billion per year.
Between 2000 and 2006, the value of subprime debt increased at an average rate of around ${{\$}} 175$ billion per year.
Work Step by Step
The average rate of change of $f$ over the interval $[a, b]$ is given as:
Average rate of change of $f\displaystyle \ \ =\frac{f(b)-f(a)}{b-a}$
The units of the average rate of change of $f$ are units of $f(x)$ per unit of $x$.
Average rate of change of $f\ \ $ over $[2,6]$ is
$\displaystyle \frac{f(6)-f(2)}{6-2}=\frac{1200-500}{4}=\frac{700}{4}=175$ billion per year.
