You can put this solution on YOUR website!
You were already given an answer to this problem, but I think the tutor accidentally hit the number 9 on his puter, instead of "0".
Let's review: The previous tutor said that if you put y = 9 into the y = mx + b format, you'd get: y = 0x + 9. AND since "m" represents the slope, you can see that the slope in this case, is a 0. (The previous tutor said the slope was 9 -- and that's a typing mistake, I'm certain).
Let's discuss slopes in general:
When x equals a number, it is a VERTICAL line and the slope is UNDEFINED.
When y equals a number, then you have a HORIZONTAL line and the slope is 0.
I tell students that the way to remember that horizontal lines have a slope of zero is to think that the word "HORIZONTAL" has two letter Os in it. I say that those Os are actually zeros, reminding you that horizontal lines have 0 as a slope. :-) Ok, corny but it works.
Good luck with your math and I hope this helps you. :-)