Find the values of $\displaystyle k$ for which the lines $\displaystyle kx-2y-1=0$ and $\displaystyle 8x-ky+3=0$ are parallel.

$\displaystyle \frac{k}{2}=\frac{8}{k}$

$\displaystyle {k^2}=16$

$\displaystyle

k=\sqrt{16}=4

$

But my textbook says it's plus or minus 4, I only got +4 why?

B.) Are there any values of $\displaystyle k$ that would make the two lines perpendicular? Explain.

How would I be able to find out?