[tex]\text{ The discriminant is 0 if the quadratic expression is a perfect square.}\\\\~~~~~b^2 -4ac =0\\\\\implies k^2-4\cdot 1 \cdot 36 = 0\\ \\\implies k^2 - 144 = 0\\\\\implies k^2 = 144\\\\\implies k = \pm\sqrt{144}\\\\\implies k= \pm12\\\\\text{Hence}~ k=12 ~\text{and}~ k=-12.[/tex]
