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If you've learned about the theorem that states that any exterior angle of a triangle is greater than any of the 2 remote interior angles, then you can solve this problem. Since you know that the exterior angle has to be greater than the interior angle, you can set up this inequality
3x - 18 > 30
3x > 48
x > 16
However, you also know that the exterior angle can't be 180, or there would be no triangle, but a line. Now you can set up a second inequality.
3x - 18 < 180
3x < 198
x < 66
By combining these two inequalities you obtain the solution, 16 < x < 66