2 answers
(click or scroll down)
f(x) = -9x
reflect in y: -9(-x) = 9x
vcomp: 1/3 (9x) = 3x
g(x) = 3x
I suspect a typo, but hey, It's not my job to figure out what you mean.
It's your job to say it so clearly you cannot be misunderstood.