If it has a single zero that means it has to be just touching the x-axis with its tip.
We know that if it has only one zero, the discriminant equals 0.
So,
[tex]D=b^2-4ac=0\implies (-k)^2-4(1)(9)=0[/tex]
Solving for k,
[tex]k=\pm\sqrt{36}=\boxed{\pm{6}}[/tex].
Hope this helps.
