I need help I don't understand it.

These angles are corresponding angles, and we know that corresponding angles are equal when a transversal intersects two parallel lines.
So,
8x - 12 = 6x + 8
=> 8x - 6x = 8 + 12
=> 2x = 20
=> x = 20/2
=> x = 10
So, the value of x is 10.
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