Answer: x = 10
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Step-by-step explanation:
The angles 4x and 3x+10 are corresponding angles. They are on the same side of the transversal cut (above the transversal) and to the right of each parallel line.
If the corresponding angles are congruent, then line A is parallel to line B.
4x = 3x+10
4x-3x = 10
x = 10