Answer: x=4
Steps: (I am assuming your question has a typo in it and by "c" you meant "x")
The rational expression equals zero
[tex]\frac{x-4}{x-6}=0[/tex]
only when the numerator equals 0 (the denominator cannot ever be zero):
[tex]x-4 = 0 \\x = 4[/tex]
and that happens only for x=4
