Answer:
The virtical angle theorem is that virtical angles are congruent, or equal. A vertical angle is like the angles in an X. The opposite angles are congruent, like in the picture, it is the unknown angle and the angle touching it.
9x+6-4x=4x+7
5x+6=4x+7
-6 -6
5x=4x+1
-4x -4x
x=1
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hope it helps
you can do this in your calculator.