A student calculating the distance between the planes ${}_1$:$-x+3y-2z+1=0$ and ${}_2$:$3x-9y+6z-3=0$

calculates the following value.

$d=\frac{|3(1)-9(0)+6(0)-3|}{\sqrt[\phantom{2}]{3^2+9^2+6^2}}=\frac{0}{\sqrt[\phantom{2}]{126}}=0$

$Is$ this correct and what $is$ the meaning?