Which value from the list below, when substituted for x, would represent an acute triangle with side lengths x, x+ 4, and 20? Assume that the longest side of the triangle is of length 20 units.

For a triangle with longest side 20, it is acute when the square of 20 is less than the sum of the squares of the other sides. With x = 14, the sides are 14, 18, and 20, and 20² = 400 < 14² + 18² = 520. These lengths also satisfy the triangle inequality, so 14 is the value that gives an acute triangle.

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