Раскроем скобки: (x^2 - 4x + 4) + (x^2 - 16x + 64) = 2x^2
2x^2 - 20x + 68 = 2x^2
-20x + 68 = 0 => 20x = 68 => x = 68/20 = 17/5 = 3.4