Question

Line CD contains points A (4, −7) and B (4, 8). The slope of line CD is

Answer 1

what is the slope called when it is 0? i thought it was called undefined

Answer 2

The slope is "undefined" when it involves division by zero. the line in question is a vertical one. On the other hand, zero divided by any number other than zero is still zero, and so you end up with a slope of zero, which characterizes a horizontal line.

I'd suggest you graph these two points and then draw a line thru them. You could then determine visually whether the line in question is a horiz. one or a vert. one.