√(2x²-14x+21) = x-4
{2x²-14x+21 = (x-4)²
{x-4 >= 0, x >= 4
2x²-14x+21 = x²-8x+16
x²-6x+5 = 0
Так как a+b+c = 0(1-6+5 = 0):
x1 = 1
x2 = c/a = 5/1 = 5
ответ: x = 5
