Answer
$$ x= 3.$$
Work Step by Step
Since $ n=\langle 1,0,0 \rangle $ and $(x_{0},y_{0},z_{0})=(3,1,-9)$
then the equation of the plane in scalar form is
$ a(x-x_{0})+b(y-y_{0})+c(z-z_{0})=0$
That is, $$(x-3)+0(y-1)+0(z+9)=0$$
and simplifying it, we get
$$ x=3$$
Hence the equation is given by
$$ x= 3.$$
