Whats (7,-4) and (7,0) in finding the slope from points

To find the slope of any line with two points given there is a specific formula you need to follow. You can apply this formula to anything.

Δy
------ or delta y over delta x
Δx
Long story short, this means the difference of the y values over the difference of the x values. With these two points given, you can find the slope through this expression:
= no slope

be mindful that no slope and zero slope are completely different. no slope is when the denominator is zero, because it is impossible for a denominator to be zero and zero slope is when the numerator is zero. Hope this helps!