Determine whether the function is linear or nonlinear. If linear, determine the equation that defines $y=f(x)$.
$$
\begin{array}{|rr|}
\hline {\boldsymbol{x}} & \boldsymbol{y}=\boldsymbol{f}(\boldsymbol{x}) \\
\hline-6 & -3 \\
-3 & -4 \\
0 & -5 \\
3 & -6 \\
6 & -7 \\
\hline
\end{array}
$$