Explanation:
f(x) = x² - 4x - 4
the general formula for solving such a quadratic equation (for f(x) = 0) is
x = (-b ± sqrt(b² - 4ac))/(2a)
in our case
a = 1
b = -4
c = -4
x = (4 ± sqrt(4² - 4×1×-4))/(2×1) = (4 ± sqrt(16 + 16))/2 =
= (4 ± sqrt(2×16))/2 = (4 ± 4×sqrt(2))/2 = 2 ± 2×sqrt(2)
x1 = 2 + 2×sqrt(2)
x2 = 2 - 2×sqrt(2)