Final answer:
The expression 3x + 21 / 6x + 6 is undefined when x is equal to -1.
Step-by-step explanation:
The expression 3x + 21 / 6x + 6 is undefined when the denominator (6x + 6) equals zero. To find the values of x for which the expression is undefined, we set the denominator equal to zero and solve for x:
6x + 6 = 0
Subtracting 6 from both sides gives:
6x = -6
Dividing both sides by 6 gives:
x = -1
Therefore, the expression is undefined when x is equal to -1. So the correct answer is A. x = -1.