Answer: D.
Steps
Equation: 9 - 3x = 0
Subtract 9 from both sides
9 - 3x - 9 = 0 - 9
Simplify
-3x = -9
Divide both sides by -3
-3x / -3 = -9 / -3
Simplify
-3x / -3
Apply the fraction rule: -a / -b = a / b
= 3x / 3
Divide the numbers: 3 / 3 = 1
= x
Note: x has the same value as 1
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Divide the numbers: -9 / -3 = 3
x = 3