f(x) = 0
x^4 + 6x^2 + 5 = 0
x^4 + 5x^2 + x^2 + 5 = 0
(x^4 + 5x^2) + (x^2 + 5) = 0
(x^2)(x^2 + 5) + (1)(x^2 + 5) = 0
(x^2 + 1)(x^2 + 5) = 0
x^2 + 1 = 0 or x^2 + 5 = 0
x^2 = -1 or x^2 = -5
x = +/-sqrt(-1) or x = +/-sqrt(-5)
x = i or x = -i or x = sqrt(5)i or x = -sqrt(5)i, where i = sqrt(-1)