SOLUTION: Solve for x. x^4 - 15x^2-16 = 0

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 = .