You mean: What is k if -3 is one of the solutions or is the only solution. Poorly formatted but I'll answer it anyway.
We have
[tex]x^2-kx-15=0[/tex].
Then substitute x with -3 and solve for k
[tex]
(-3)^2-k(-3)-15=0 \\
9+3k-15=0 \\
3k=6\Longrightarrow k=\dfrac{6}{3}=\boxed{2}
[/tex]
Hope this helps.
