
Kenneth S. answered 03/21/16
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Expert Help in Algebra/Trig/(Pre)calculus to Guarantee Success in 2018
You have no figure associated with the given equation.
Taking square roots of each side of your equation, you get x+12 = √(400)
so x+12 = 20 so x = 8 (I'm ignoring the extraneous answer based on -20 on right side since this is Geometry).