3x^4 - 2x^2 = 16. Introduce new variable t = x^2. Then your equation takes the form 3t^2 - 2t - 16 = 0. Solve it using the quadratic formula= = = . Only positive root is acceptable (since t = x^2), t = = = . It implies x^2 = ; hence, x = +/- = +/- . ANSWER