Hi
x^4-16x^2+48
X^2 - 16X + 48 = 0          | Let  X = x^2 
(X - 12)(X - 4) = 0
 X = 12      0r   X = 4
 x^2 = 12         x^2 = 4                
  x = ± 2√3         x = ± 2
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