If x + y = k is a normal to the parabola y² = 12x, then k is:

Any normal to the parabola $y^2=12x$ is $y+tx=6t+3t^3$. It is  identical with x + y = k if $\frac{t}{1}=\frac{1}{1}=\frac{6t+3t^3}{k}$

$\therefore t=1$ and $1=\frac{6+3}{k}\Rightarrow k=9$