Answer
The rate of change is not constant, the data in the table cannot be modeled by a linear equation.
Work Step by Step
The rate of change for consecutive data pairs in the table.
$\Rightarrow \frac{15-18}{2-1}=\frac{-3}{1}=-3$
$\Rightarrow \frac{12-15}{4-2}=-\frac{3}{2}=-1.5$
$\Rightarrow \frac{9-12}{8-4}=-\frac{3}{4}=-0.75$
Because the rate of change is not constant, the data in the table cannot be modeled by a linear equation.
