Simplify x/6x-x^2

Question

I know that the answer is 1/6-x but the answers look like this:
A. 1/6-x where x is not equal to 0,6
B. 1/6-x where x is not equal to 6

Please help me with understand why is says x is not equal to, I have like 5 problems just like this so I really need to understand this, thanks so much.

Answer 1

the denominator is 6x-x^2 = x(6-x)
division by zero is not defined, and since the denominator is zero at x=0,6
those two values must be excluded from consideration. So, A

Answer 2

Thank you, so is simplify 5x^3/7x^3+x^4 = 5/7+x where x is not equal to 0,-7?

Answer 3

yes, as I noted in your earlier post.