Since, the line AB is parallel to X - axes, so every point on the the line AB will have equal Y - coordinates.
lets take it as Y ( y - coordinate is same )
slope :
=》
[tex] \dfrac{y2 - y1}{x2 - x1} [/tex]
but since y2 and y1 are equal, so
[tex] \dfrac{ Y - Y }{x2 - x1} [/tex]
=》
[tex] \dfrac{0}{x2 - x1} [/tex]
=》
[tex]0[/tex]
