[tex]\bf \begin{cases}
f(x)=-x-2\\
g(x)=x^2\\
(g\circ f)(-6)\implies g[~~f(-6)~~]
\end{cases}
\\\\\\
\textit{now, what is f(-6)?}\qquad f(-6)=-\stackrel{x}{(-6)}-2\implies f(-6)=6-2
\\\\\\
\boxed{f(-6)=4}
\\\\\\
g[~~f(-6)~~]\implies g[~~4~~]=(4)^2[/tex]