Which represents all the values of x that make the rational expression -6x + 42/3x + 9 undefined?

Given that the division by zero is undefined, the rational expression will be undefined where its denominator is equal to zero.

So, you need to determine for which x-values 3x + 9 = 0

=> 3x = - 9

=> x = - 9 / 3

=> x = - 3

- 3 is all and the only value of x that make the rational expression given undefined.