In a parallelogram, opposite angles are equal and adjacent angles are supplementary (i.e., they add up to 180 degrees).
In this case, angle B and angle D are adjacent angles. Therefore, we can set up the equation:
\[ (3x - 7) + (x + 15) = 180 \]
Now, simplify and solve for \(x\):
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Combine like terms: \[ 3x - 7 + x + 15 = 180 \] \[ 4x + 8 = 180 \]
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Subtract 8 from both sides: \[ 4x = 172 \]
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Divide by 4: \[ x = 43 \]
So the value of \(x\) is 43.