Question

What is the slope of the line x=-6!? What about y=6 Please explain :)

Answer 1

This is super simple. :)
If it is asking for a x=a it will always be 0 because there is no y intercept. It is a horizontal line.
For y=6 is undefined because it is a straight verticle line. An easy way to remember is if it is y or x = something with just 1 number and no coefficents on that number it will be x=0 or y=undefined. Also try to picture how it would graph and it might help. :)

Answer 2

So,

These are actually special cases.
x = -6

No matter what value we substitute for y, x will always be -6.
These are some solutions to it:
(-6, 0)
(-6, -2)
(-6, 50)

Now, we know that:
$slope(m) = \frac{rise}{run}$

But the run is zero, because the line is vertical.  And dividing by zero is not allowed.  Therefore, the slope of the line x = -6 is undefined.

y = 6

This is basically the same type as the previous one, with one exception.

Some solutions:
(0, 6)
(-47, 6)
(59, 6)

No matter what value we substitute for x, y will always be 6.

Remembering what the slope is equal to, we quickly see that since the rise is zero, and the denominator is infinite, we can conclude that the slope is zero.