Geometry

Question Analysis：  
This problem uses the midsegment formula with all sides expressed as variables.

Key Concept Explanation：  
In any trapezoid,  
Midsegment = (Base₁ + Base₂) ÷ 2

Step-by-Step Solution：
Set up the equation:  
6x - 9 = [(x + 9) + (5x - 3)]/2
6x - 9 = (6x + 6)/2
6x - 9 = 3x + 3
3x = 12
x = 4

Common Mistakes：