Values of $k$ for which the line $y=kx-1$ is tangent to the parabola with the equation $y=x^{2}+3$ How can I find the values of $k$ for which the straight line $y=kx-1$ is tangent to the parabola with the equation $y=x^{2}+3$?   
I used this short cut form $c=-am^{2}$, which gives me $k=\pm 2$. I think I am not correct. What's the answer?

HINT:
Find the abscissa $(x)$ of the intersections by equating the values of $y$
Now the two values must be same for tangency
