If the slope of a line is zero, I am assuming x=-4 has a slope of zero, since it is a straight line, does that make it undefined?
All horizontal lines have slope 0. And if any line has slope 0, it must be a horizontal line.... A line may have a positive slope, negative slope, zero slope, or undefined slope. Now, i am very visual and this helps me to remember ...if you were walking on the lines of the actual graph itself (lft to rt), then remember it this way ..If you are: walking UP hill: it is a positive slope ...walking DOWN hill: it is a negative slope and walking a FLAT line: well, it is a zero slope with no hill at all ... Now, for a slope to be undefined, it MUST be vertical ..
ONLY if the line is PERFECTLY VERTICAL is it said to have no slope, or to be
an undefined slope... hope this helps!