Answer
(a) Dependent variable - $N$; Independent variable - $r$
(b) There were $13.49$ million housing units in $2012$ that had $3$ rooms.
Work Step by Step
$N(r) = - 1.35r^2 + 15.45r - 20.71$
(a) Dependent variable - $N$; Independent variable - $r$
(b) Substitute $r=3$. This will yield the number of housing units (in millions) in $2012$ that had $3$ rooms.
$N(r) = - 1.35r^2 + 15.45r - 20.71$
$N(3) = - 1.35(3)^2 + 15.45(3) - 20.71$
$N(3)=13.49$
Therefore, there were $13.49$ million housing units in $2012$ that had $3$ rooms.
