In a parallelogram, opposite angles are equal. Since the top middle angle is \( 2x + 1 \) and the bottom middle angle is \( 21 \), we can set up the equation:
\[ 2x + 1 = 21 \]
To solve for \( x \), follow these steps:
- Subtract 1 from both sides:
\[ 2x = 21 - 1 \] \[ 2x = 20 \]
- Now divide both sides by 2:
\[ x = \frac{20}{2} \] \[ x = 10 \]
So the value of \( x \) is \( 10 \).
The answer is \( \boxed{10} \).