Relating the coordinates, it is found that the coordinate of B is -2.
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The distance between A and B is 1/3 of AC, thus:
[tex]x + 9 = \frac{21}{3}[/tex]
[tex]x + 9 = 7[/tex]
[tex]x = -2[/tex]
The coordinate of B is -2.
A similar problem is given at https://brainly.com/question/18285191