Ignoring the "?" in your question:
[tex]5k+10k+6=0 \iff 15k+6=0 \iff k=-\dfrac{6}{15}[/tex]
If the "?" is a squared symbol that wasn't caught:
[tex]5k^2+10k+6=0[/tex]
Use the quadratic formula
[tex]x_{1,2}=\dfrac{-b\pm\sqrt{b^2-4ac}}{2a}[/tex]
to get the solutions:
[tex]x_{1,2}=\dfrac{-10\pm\sqrt{100-120}}{10}[/tex]
as you can see, the discriminant is negative, so the equation has no solution.
