Since angles \( \angle 1 \) and \( \angle 3 \) are vertical angles, they are equal. Therefore, we can set their measures equal to each other.
We know:
- \( m\angle 1 = 3x - 25 \)
- \( m\angle 3 = 104 \)
Setting them equal gives us the equation:
\[ 3x - 25 = 104 \]
To solve for \( x \), first add 25 to both sides:
\[ 3x = 104 + 25 \]
\[ 3x = 129 \]
Next, divide both sides by 3:
\[ x = \frac{129}{3} = 43 \]
Thus, the value of \( x \) is \( \boxed{43} \).