Geometry

Question Analysis：
This question uses the midsegment property of trapezoids.
The focus is on solving an equation using the average of the two bases.

Key Concept Explanation：
In a trapezoid, the midsegment equals the average of the two bases:
EF = (AB + CD) ÷ 2

Step-by-Step Solution：
Given:
EF = 31, AB = 3x + 7, CD = 5x - 1
Set up the equation:
31 = (3x + 7 + 5x - 1) ÷ 2
31 = (8x + 6) ÷ 2
31 = 4x + 3
28 = 4x
x = 7

Common Mistake...