Applying the same-side interior angles theorem, the value of x is: D. 20.
<h3>What is the Same-Side Interior Angles Theorem?</h3>
The same-side interior angles are said to be supplementary to each other if they lie on two parallel lines, according to the same-side interior angles theorem.
Given that lines a and b are parallel, the two angles shown in the image given are same-side interior angles, therefore:
6x + 3x = 180 [according to the same-side interior angles theorem]
Combine like terms
9x = 180
Divide both sides by 9
9x/9 = 180/9 [division property of equality]
x = 20
The value of x is therefore: D. 20.
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