Geometry

Question Analysis：
This question applies the midsegment formula where one base is an expression.
The focus is solving for x from an average equation.

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

Step-by-Step Solution：
Midsegment = 15
Base₁ = 2x + 6, Base₂ = 18
15 = (2x + 6 + 18) ÷ 2
15 = (2x + 24) ÷ 2
15 = x + 12
x = 3

Common Mistakes：
Forgetting to divide the sum of the ba...