Question

Kahn Academy Question, Please help!

Answer 1

$g\circ f(-9)=g(f(-9))$

By definition of $f$,

$f(-9)=3\cdot9-8=19$

and by definition of $g$,

$g\circ f(-9)=g(f(-9))=g(19)=\sqrt{14-19}-10=\sqrt{-5}-10$

which is undefined if $f,g$ are supposed to be real-valued functions. If they're complex-valued, then $\sqrt{-5}=i\sqrt 5$ and $g\circ f(-9)=-10+i\sqrt 5$.