A triangle has side lengths of 12, 15, and x. The value of x must ne greater than what?

  3 < x < 27

Step-by-step explanation:

We can't tell what "must ne greater" means, so we'll tell you the limits on the side length and you can choose the one that is relevant.

The third side of a triangle must have a length that lies between the difference and sum of the two lengths you know.

 15 -12 = 3

  15 +12 = 27

The value of x must be between 3 and 27:

  3 < x < 27

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If you allow the triangle to have zero area, then x can be 3 or 27. Many authors claim such a "triangle" is not allowed, so don't allow those cases.