Geometry

Question Analysis：
This problem tests your understanding of how to apply the side properties of parallelograms.
The main focus is on recognizing and solving equations from equal opposite sides.

Key Concept Explanation：
In parallelograms, opposite sides are equal in length.
This allows us to form and solve an algebraic equation.

Step-by-Step Solution：
Set up the equation: 3x + 1 = x + 11
Subtract x from both sides: 2x + 1 = 11
Subtract 1: 2x = 10
Divide by 2: x = 5

Common Mistakes：
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