Is this explanation correct?

Question

quickstudent · Answer

question: State whether the slope of a line passing through (2, -3) and (1, 8) is positive, negative, zero or undefined.

my answer: This line has a negative slope, because the values of the y coordinates decrease from left to right.

quickstudent · Answer

@Astrophysics @Evoker

photon336 · Answer

hey @quickstudent two of those answer choices right away can be eliminated.

quickstudent · Answer

What answer choices? I wrote one answer.

photon336 · Answer

yes you did :) this is what i'm referring to "positive, negative, zero or undefined."

we know that these two points are a line so the slope cannot be zero or undefined. 

$$y  = m(x)+b$$

$$m = \frac{ y_{2}-y_{1} }{ x_{2}-x_{1} }$$ 

$$\frac{ 2-1 }{ -3-8 }$$ 

yep  :) 

$$\frac{ 1 }{ -11 }$$

quickstudent · Answer

Oh, OK:)    Thanks:)

photon336 · Answer

yeah, what I was trying to say is that if you've got a straight line it's going to be defined for all real numbers unless there are discontinuities or things like that.