Respuesta :

calculista
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



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