Since angles ∠1 and ∠3 are a vertical pair, they are equal. Therefore, we can set up the equation:
\[ m∠1 = m∠3 \]
Substituting the given expressions:
\[ 3x - 25 = 104 \]
To solve for \(x\), first add 25 to both sides:
\[ 3x = 104 + 25 \] \[ 3x = 129 \]
Next, divide both sides by 3:
\[ x = \frac{129}{3} \] \[ x = 43 \]
So the value of \(x\) is \( \boxed{43} \).