Since ∠1 and ∠3 are a vertical pair, they are equal in measure. Therefore, we can set up the equation:
\[ m∠1 = m∠3 \]
Substituting the given expressions:
\[ 3x - 25 = 104 \]
Now, let's solve for \(x\):
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Add 25 to both sides of the equation:
\[ 3x - 25 + 25 = 104 + 25 \]
\[ 3x = 129 \]
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Divide both sides by 3:
\[ x = \frac{129}{3} \]
\[ x = 43 \]
Therefore, the value of \(x\) is \( \boxed{43} \).