If the slope is #0# then by definition this is a horizontal line and for and all values of #x# (including when #x = 0#) y is the same. And, because we were given the y-intercept as #5# we know the equation for this line is:
#y = 5#
Explanation:
If the slope is #0# then by definition this is a horizontal line and for and all values of #x# (including when #x = 0#) y is the same. And, because we were given the y-intercept as #5# we know the equation for this line is: