Your starting equation is x^4 - 15x^2-16 = 0 It is so called biquadratic equation. To solve it, factor left side (x^2 - 16)*(x^2 + 1) = 0. In real domain, it gives two solutions x= 4 and x= -4. In complex numbers, there are 4 possible solutions: x= 4, x= -4, x= i =and x= -i = .