Recall that the general point-slope form is where m is the slope and is the point the line goes through. We see that the 2 and the m match up, so m = 2 is the slope.
Side note: We can rewrite that given equation into y - (-5) = 2(x - (-1)) to see that the line goes through the point (-1, -5)
You can look at it like a right triangle being made with the vertical part being 9 and the horizontal part being 8, then you want to find the diagonal part. So that's the Pythagorean theorem