Answer
$\frac{\sqrt {14}}{42}$
Work Step by Step
Distance formula between two parallel planes is given as:
$D=\frac{|ax+by+cz+d|}{\sqrt {a^2+b^2+c^2}}$
$D=\frac{|1-0+0-0|}{\sqrt {3^2+6^2+9^2}}=\frac{1}{\sqrt {126}}$
$=\frac{\sqrt {14}}{42}$