The slope of the line that passes through the points (2,5) and (-1,5) is 0: it is an horizontal line, because both points have the same y-coordinate. The equation of the line is y=5.
For any two points (x1,y1), (x2,y2), the slope, m, of the line that passes through them can be calculated with:
[tex]m= \dfrac{\Delta y}{\Delta x} = \dfrac{y2-y1}{x2-x1} [/tex]
where [tex]\Delta y,\ \Delta x[/tex] represent the increment on the y direction and the increment on the x direction, respectively.
This comes from the definition of "slope": the slope of a line tells you how much does the line grow in the vertical direction for every unit of advance in the x direction.
If you apply the formula to the points given, you get [tex]\Delta y=5-5=0[/tex].
