Because both points have the same #y# value of #-8# we know by definition this is a horizontal line with a slope of #m = 0#.
The point-slope formula states: #(y - color(red)(y_1)) = color(blue)(m)(x - color(red)(x_1))#
Where #color(blue)(m)# is the slope and #color(red)(((x_1, y_1)))# is a point the line passes through.
Substituting the slope and the first point from the problem gives:
#(y - color(red)(-8)) = color(blue)(0)(x - color(red)(5))#
#(y + color(red)(8)) = color(blue)(0)(x - color(red)(5))#
We can also substitute the slope and the second point from the problem giving:
#(y - color(red)(-8)) = color(blue)(0)(x - color(red)(-9))#
#(y + color(red)(8)) = color(blue)(0)(x + color(red)(9))#
