we know that
slope m=(y2-y1)/(x2-x1)
then
case a) x=18
m=(y2-y1)/(18-18)--------------> is undefined, because the denominator is zero
case b) 2y-6x=0
2y=6x------------> y=3x
the slope m=3-----------> m is defined
case c) y=9
m=(9-9)/(x2-x1)--------> m=0-----------> m is defined
case d) y=x
m=1---------------> m is defined
the answer is
x=18 has an undefined slope