Nice question.

Start with 4x - 12>x + 9

Subtract x from both sides to get: 3x - 12>9

Add 12 to both sides to get: 3x>21

Divide both sides by 3 to get:x>7

The correct answer isSPOILER: A

Here's why:

Ifx,then x MUST be greater than 6 (and the question asks what MUST be true?)>7

Many students will choose answer choice B, because it looks very similar tox>7

However, ifxthen x COULD>7equal7, but answer choice B says that x must begreater than7

So, answer choice B need not be true

Cheers,

Brent